John, B., Hewitt, G.M.: The B-chromosome system of Myrmeleotettix ... John, B., Lewis, K.R.: Chromosome variability and geographical distribution in insects.
Chromosoma (Berl.) 71, 371 -386 (1979)
CHROMOSOMA 9 by Springer-Verlag 1979
Chromosome Polymorphism in Populations of the Grasshopper Trimerotropis pallidipennis from Southern Argentina Ekaterina S. de Vaio, Beatriz Gofii and Cristina Rey Departamento de Gen6tica, Instituto de Biociencias, Facultad de Humanidades y Ciencias, Universidad Mayor de la Repflblica, Cerrito 73, Montevideo, R.O. del Uruguay
Abstract. Three populations of the grasshopper Trimerotropis pallidipennis from southern Argentina have been studied cytologically. A very characteristic B-chromosome was found in all three. They also showed geographical variability in respect of the presence of pericentric inversions, and the inversion system was found to influence chiasma frequency. The Laguna Blanca population, which is on the hypothetical pathway the species is believed to have followed during its migration from northern to southern latitudes, has the same karyotype composition as the N. American form, with fixed inversions in the 3 largest autosomes and the X-chromosome. Its members have a high total chiasma frequency and a great number of interstitial chiasmata. The Sierra de la Ventana population, situated at the absolute eastern border of the species distribution is highly polymorphic with respect to the presence of inversions in the medium chromosomes. Its members have the lowest total chiasma frequency and a greatly reduced number of interstitial chiasmata. Situated geographically between the other two, the ChoeleChoel population has the highest frequency of inversions and many of them are homozygous, its members have a higher total chiasma frequency than that observed in specimens from Sierra de la Ventana, and a greatly reduced number of interstitial chiasmata, similar to that observed in individuals from the latter population.
Introduction
Cytological interest in species of the genus Trimerotropis, and the closely related genera Circotettix and Aerochoreutes, lies in the fact that all three genera show chromosomal variability with respect to the position of the centromere (White, 1945). Pericentric inversions are generally considered to cause this variation although the possibility that it involves centric transpositions cannot be excluded (Jackson, 1973). These structural changes have been considered to be pericentric inversions for the purpose of this paper. In some cases one or more of these 0009-5915/79/0071/0371/$03.20
372
E.S. de Vaio et al.
structural rearrangements have became fixed in a population, in others they occur in a polymorphic state (Carothers, 1917, 1921, 1926, 1931; Coleman, 1948; Evans, 1954; Helwig, 1929, 1955; John and Weissman, 1977; King, 1924; Schroeter, 1968; Weissman, 1976; Wenrich, 1917; White, 1949, 1951a, 1951b, 1951c; White and Morley, 1955). Most species of the genus Trimerotropis are found in North America with only a few having been described for the neotropical region (Mesa, 1971). Trimerotropis pallidipennis is found in the United States and South America where it follows the Andes and invades adjacent arid or semiarid regions (Carbonell, personal communication). Some of its chromosomes are metacentric and thus it belongs to section " B " of the genus (White, 1949). The chromosomes of the North American populations of T. p. pallidipennis are structurally homozygous (White, 1949; Coleman, 1948; Weissman, personal correspondence). The southern populations, however, are polymorphic for centromere position and for the presence of a B-chromosome (Mesa, 1971; Mesa and Vaio, 1976). The adaptive significance of chromosomal rearrangements may depend on their effects on recombination (John and Lewis, 1966) and such effects certainly occur in the Trimerotropines (White, 1951, a, b). Patterned chromosome variation influencing chiasma frequency has also been demonstrated for T. helferi (Schroeter, 1968) and for T. pseudofasicata (Weissman, 1976). The present paper describes the polymorphic chromosomal constitution of three populations of T. pallidipennis from southern Argentina, ranging over a linear distance of at least 300 kilometers. We have found that this polymorphism shows geographical variation and has a significant influence on chiasma frequency.
Material and Methods The 50 specimens studied in this paper were part of 300 individuals collected by Dr. A. Mesa and Lic. E. Corbella in Argentina in 1973, and identified by Dr. C.S. Carbonell. These 50 males came from the following localities: Sierra de la Ventana, Province of Buenos Aires, 21; 70 kmts. East of Choele-Choel, Province of Rio Negro, 19 ; L a g u n a Blanca, Province of Neuquen, 10 (Fig. 1). Testes were removed in the field and fixed in alcohol-acetic acid (3: 1). Before making squash preparations, the material was softened in 45% acetic acid (aqueous). Staining was with lacto-acetic orcein. Chiasma frequency was studied in 10 metaphase-I nuclei per individual. Total chiasma frequency includes both terminal and interstitial chiasmata. The n u m b e r of interstitial chiasmata was registered separately for the same ten nuclei (Zarchi et al., 1972).
Observations
A. Systematics A m o n g the neotropical Acridoidea, the Oedipodinae are invaders from the neartic region. In South America they are a m o n g the most recent invaders, as is evidenced by the small number of genera and species found in this subcontinent and by the fact that, with the sole exception of Trimerotropis, they are restricted to the northern part of South America. Another indication of the recent migration
Chromosome Polymorphism in Trimerotropis pallMipennis
r. o
-,&
,;o " t . '
Santiago [
373
f-
Buenos Aires ?Montevideo
, '36~
,~
['~4~
.... 5"o
Sierra de la Ventana
p
Choele-ehoel
'4~176
i
Fig. 1. Map showing the three localities where the 50 male Trimerotropis patlidipennis were collected
of this group to South America lies in the fact that the species T. pallidipennis is the same one found in the northern subcontinent (Carbonell, unpublished). T. pallidipennis extends from southern Canada, over all western U.S.A. and Mexico. It is absent from Central America; and southward it is found all along the Andes, in Peru, Ecuador, Bolivia, Chile, and Argentina. In South America it shows an amazing plasticity in adaptation to ecological and altitudinal situations. The only basic requirement for its habitat seems to be the prevalence of arid or semiarid conditions (Carbonell, unpublished). According to Rehn (1940) there are two subspecies of T. pallidipennis in South America: andeana, confined to a restricted area of relatively high altitude in the Peruvian-Bolivian '~altiplano ", and pallidipennis, found in the rest of the southern subcontinent. Following Rehns' distribution our specimens must be considered to be T. p. pallidipennis (Carbonell, unpublished). Carbonell (unpublished) suggests that the territory assigned by Rehn to the subspecies andeana is not isolated by any type of barrier from that occupied by the species as a whole, and what Rehn considers to be a subspecies is probably just a variant correlated with high altitude. In the series collected by Dr. Carbonell and Dr. Mesa it has proven difficult to separate 7", p. andeana from T. p, pallidipennis because of the presence of nmnerous intermediate forms (Carbonell, unpublished),
B. The K a r y o t y p e The c h r o m o s o m e n u m b e r of T. patlidipennis is 2nc~ = 2 3 , with a n XOd', X X 9 sex m e c h a n i s m . The a u t o s o m e s can be arbitrarily g r o u p e d into three large (L1, L2, L3), five m e d i u m (M4, Ms, M6, My, M8) a n d three small pairs ($9,
Slo, Si0. T h e L~, L2 a n d L3 are always s u b m e t a c e n t r i c t h r o u g h o u t the species' range, a n d the three small pairs are i n v a r i a b l y rod like. I n L a g u n a Blanca specimens, the five m e d i u m pairs are all r o d s ; individuals from the other two p o p u l a t i o n s , however, show changes in c e n t r o m e r e position. F o u r of the five m e d i u m pairs were difficult to distinguish from each other consistently (Ms, M6, My, Ms) so the fi'equencies of inversions given in T a b l e 1, a n d their descriptions, m u s t
374
E.S. de Vaio et al.
Table 1. Frequency of the different M-inversions morphs in two argentinian populations of T. pallidipennis a
Locality
M4r
M4 r M5 T M5 M MAT M6 s M6 M My1. M7 s M,~, M8 T Mss M8~
Sierra de laVentana Choele-Choel
0.62 0.11
0.38 0.89
0.95 0.97
0.05 0.03
0.52 0.44
0.17 0.00
0.31 0.66
0.45 0.03
0.19 0.34
0.36 0.63
0.33 0.12 0.14 0.13
0.55 0.73
A = acrocentric, T = telocentric, M = metacentric, S - submetacentric
Fig. 2a-f. First metaphase plates of males of T. pallidipennis from Sierra de la Ventana (a-d and f) and Laguna Blanca (e). Note terminal neocentric activity of the X-chromosome in d and e. D Ditactic bivalent; H heteromorphic bivalent; D H double heteromorphic bivalent; and I H inversion homozygote bivalent. Arrows indicate secondary constrictions, a (individual 208) 17 Xa, 1 interstitial; b (individual 192) 16 Xa, 2 interstitial; c (individual 234) 16 Xa, 2 interstitial; d (individual 233) 15 Xa, 0 interstitial; e (individual 546) 22 Xa, 9 interstitial; f (individual 211) 16 Xa, 1 interstitial
Chromosome Polymorphism in Trimerotropis pallidipennis"
375
Fig. 3a-f. First metaphase plates of males of T. paltidipennis from Sierra de la Ventana (a-c), Choele-Choel (d-e) and Laguna Blanca (f). Note terminal neocentric activity of the X-chromosome in b. D, H, DH and IH as in Fig. 2. Arrows indicate secondary constrictions, a (individual 218) 13 Xa, 1 interstitial; b (individual 209) 16 Xa, 0 interstitial; c (individual 226) 16 Xa, 1 interstitial; d (individual 378) 14 Xa, 0 interstitial; e (individual 346) 16 Xa, I interstitial; f (individual 536) 19 Xa, 6 interstitial
be regarded as tentative. As none of the individuals from Sierra de la Ventana and Choele-Choel show more than three of the medium sized pairs with inversions, the possibility exists that one of these pairs (Ms Ms) may never be inverted. The rod like chromosomes in the Trimerotropines have always been consid-
E.S. de Vaio et al.
376
ered by White (1973) to be acrocentric but most of them have been demonstrated to be telocentric (John and Weissman, 1977). We shall refer to them as telocentric for the purpose of this paper. The X chromosome is large and metacentric, and it may sometimes show terminal neocentric activity, adopting the shape of a telocentric element (Fig. 2 e). Secondary constrictions are generally seen in the $9 metaphase-I bivalent, but there is heteromorphism for their expression so that some nuclei in every individual from all three populations, show only one such constriction (Figs. 2a, and 3 b, d). Other heteromorphic secondary constrictions were observed in specimens from Sierra de la Ventana in some of the medium sized pairs (Figs. 2a, f and 3a, b) and in both $10 and $11 (Fig. 2c, d). The possibility that they represent inversion homozygotes in which one of the arms has failed to flex at the centromere, was discarded by studying other metaphse-I plates in the same individual. A very characteristic telocentric B-chromosome was observed in some individuals of all three populations (Fig. 3 a, c). It is approximately the size of the $9 bivalent and, at first metaphase, it has an isopycnotic proximal region and a negatively heteropycnotic distal region.
Table 2. Chiasma characteristics of the three argentinian populations of T. pallidipennis Population: Sierra de la Ventana Individual, no. and type
Mean cell chiasma frequency
Mean no. interstitial chiasmata per cell
218- het. M4,M6,Mv,M s B-chr. 222- bet. M4,M6,Mv inv.hom. Ms 233- het. M6,M 7 200- her. M4,M6,Ms inv.hom. M7 226- het. M4,M 7 inv.hom. Ms B-chr. 199- het. Mg,M 6 inv.hom. Ms 189-het. M4, M v inv.hom. M6,Ms B-chr. 234- het. M4,M6,Mv 201- het. M4,MT,Dhet. M 6 inv.hom. Ms 183- het. M4,M6,MT,M s 230- het. M5,M 6 inv.hom. M7 B-chr. 194- het. M4,M6,M7 B-chr. 209- het. M6,M v inv.hom. M s 208- her. M4,M 6 inv.hom. M s 192- het. M6,MT,Ms 211- het. M6,Ms 223- inv.hom. M7 B-chr. 196- bet. M6 inv.hom. MT,M s 185- het. M~,Ms,M7 236- het. M6,MT,M8 190- het. M6,M7 inv.hom. M 8
14.1 14.2 14.2 14.7 14.7 15.0 15.2 15.3 15.5 15.5 15.6 15.7 15.7 15.9 16.1 16.1 16.5 16.6 16.8 17.0 17.1
0.8 1.8 0.3 1.5 1.2 1.6 1.3 1.8 0.7 0.6 3.2 2.1 0.4 1.9 1.2 2.2 1.8 2.8 1.2 1.5 1.3
15.60_+0.89
1.49+0.71
Population means inv. per male 4.10 (2 6) het. biv. per male 2.43 (0-4)
Chromosome Polymorphism in
Trimerotropispallid•
377
Table 2 (continued) Population : Choele-Choel Individual, no. and type
Mean cell chiasma frequency
Mean no. interstitial chiasmata per cell
378- net. M6, Dhet. M 7 inv.hom. M8 357- het. M6 inv.hom. M7M 8 346- het. M4,M6,MT,Dhet. M s 344- het. M6 inv.hom. Mv,M s 350- het. Mr inv.hom. M7,Ms B-chr. 353- inv.hom. M6,MT,M 8 380- het. M6 inv.hom. MT,M8 383- bet. Ms,M6 inv.hom. M 7 377- het. M6 inv.hom. MT,M s 347- het. M6 inv.hom. MT,Ms 343- het. M6 inv.hom. MT,M s B-chr. 349- het. Ms inv.hom. M6,M7 376- het. M6 inv.hom. MT,M s 374- inv.hom. M6,MT,M 8 379- het. M6,M8 inv.hom. M7 375- inv.hom. M6,MT,Ma 361- bet. M6 inv.hom. M7,M s 375- het. M8 inv.hom. M6,M 7 358- inv.hom. M6,M7,M s
15.2 15.5 15.7 16.2 16.4 17.0 17.2 17.2 17.5 17.5 17.6 17.7 I7.8 17.9 18.2 18.2 18.4 18.8 19.3
1.3 0.6 0.8 0.2 1.1 1.2 1.7 1.3 2.1 2.2 0.4 1.5 0.9 2.5 1.7 0.6 1.1 1.3 0.9
17.33 • 1.08
1.23 +_0.60
Individual, no. and type
Mean cell chiasma frequency
Mean no. interstitial chiasmata per cell
546536- B-chr, 532- B-chr, 545555552547538526535-
18.5 19.1 19.6 19.9 20.2 20.4 21.1 21.6 21.7 22.1
6.8 5.0 7.4 7.7 7.7 5.4 7.5 7.6 9.4 8.9
20.42 • 1. i8
7.34 _+1.29
Population means inv. per male 5.16 (4-6) het. biv. per male 1.16 (0-4) Population: Laguna Blanca
Population means
T h e l a r g e s t m e d i u m a u t o s o m a l p a i r ( M ~ ) is easily d i s t i n g u i s h e d b y its l e n g t h . A t f i r s t m e t a p h a s e it f r e q u e n t l y f o r m s t w o c h i a s m a t a , o n e in t h e c e n t r o m e r e a n d o n e d i s t a l l y in t h e l o n g a r m , g i v i n g rise t o a d i t a c t i c b i v a l e n t ( F i g s . 2 a , b, c, d, f a n d 3 a - e ) . It a l s o m a y h a v e a v e r y l o n g i n v e r s i o n , w h i c h s h i f t s the c e n t r o m e r e to the o t h e r e n d o f the c h r o m o s o m e . This inversion results
378
E.S. de Vaio et al.
Table 3. Climatic characteristics of the three localities sampled in southern Argentina (Montevideo as reference) Climatic elements
Sierra de la Ventana
Choele-Choel
Annual isotherms
15-16 ~ C
15-16 ~ C
8-9 ~ C
16.5 ~ C
January isotherms
23 ~ C
23-24 ~ C
18-19 ~ C
22.5 ~ C
6.5 ~ C
4-5 ~ C
I0.5 ~ C
July isotherms
5-7 ~ C
L. Blanca
Montevideo
N ~ of frost days/year
35-40
55-60
over 60
25
Mean January relative humidity
50-55%
45%
50-55%
71%
Mean July relative humidity
70%
65%
70-75
75%
Mean January precipitation
40-60 m m
20-40 mm
20-40 m m
170 mm
Mean July precipitation
20 mm
up to 20 m m
20-40 mm
70 mm
in an acrocentric chromosome, easily recognized in heterozygous bivalents because its short arm associates by a terminal chiasma with the distal end of the long arm of its homologue (Figs. 2c and 3 c, e). The inversion homozygote cannot be distinguished from the unmodified pair at metaphase I. The M5 has an inversion that renders it metacentric in both the Sierra de la Ventana and Choele-Choel populations. The other medium sized pairs (M6, My, M8) have three different structural forms in Sierra de la Ventana where they may be telocentric (T), submetacentric (S) or metacentric (M), and almost all possible combinations of them are present in this population, including SM double heterozygotes (Fig. 3d, e). A submetacentric M6 was not found in Choele-Choel, but the three structural forms were observed in both M7 and Ms, homozygous combinations predominating (see Table 2).
C. Climatic Data
The climatic characteristics of the three localities are found in Table 3. Laguna Blanca, with the coldest climate, situated at the "precordillera", would be in the hypothetical pathway of the species migration toward the South. Sierra de la Ventana probably has the mildest climate, the most abundant rainfall and is at the absolute eastern border of the species range. Choele-Choel is located approximately halfway between the other two localities in a typically desert area.
Chromosome Polymorphism in Trimerotropispallidipennis 4. F values for the analysis of variance comparing the populations at Sierra de la Ventana and Choele-Choel
Table
Table 5. F values for the analysis of variance comparing the three argentinian population of
Trimerotropispallidipennis
379
Number of Inversions
Heteromorphic bivalents
Homozygous bivalents for the inversion
F=18.42 P 0.10
P>0.10
ChoeleChoel
- F ( 1 , 1 7 ) - 5.85 F(1,17) =0.16 0.05>P>0.01 P>0.10
-F(1,17)=0.18 P>0.10
--F(1,17) =0.06 P>0.10
F(1,17)=0.87 P>0.10
--
F(I,8)= 10.32 P>0.01
Laguna Blanca
Number of inversions per male vs. interstitial chiasma frequency
-
F(1,19)
=0.001
Interstitial chiasma frequency vs. total chiasma frequency
- F(1,19)
=
1.68
Table 7. Regression analysis comparing the data of all the individuals of T. pallidipennis studied. (The sign preceding F indicates the slope of the regression line) Number of heteromorphic bivalents per male vs. total chiasma frequency
Number of inversions per male vs. total chiasma frequency
Number of heteromorphic bivalents per male vs. interstitial chiasma frequency
Number of inversions per male vs. interstitial chiasma frequency
-F=58.00 P < 0.001
-F=20.98 P < 0.001
-F=24.08 P < 0.001
-F=117.91 P < 0.001
6 and 7. When both polymorphic populations were studied separately (Table 6), only one significant relationship was found: in Choele-Choel the number of heteromorphic pairs per male (independent variable) versus total chiasma frequency (dependent variable), shows a significant inverse relationship in seven of the eight analyses. The regression analysis, combining the data from all 50 individuals, demonstrates a highly significant inverse relationship in every case (Table 7).
F. B-Chromosome
A very characteristic B-chromosome with a distal heteropycnotic segment and a proximal isopycnotic one was observed in 6 of the 21 specimens from Sierra de la Ventana, 3 of the 19 specimens from Choele-Choel and 2 of the 10 specimens from Eaguna Blanca. To investigate if this supernumerary chromo-
Chromosome Polymorphism in Trimerotropis pallidipennis
381
some segregated in a particular way, we assumed the X chromosome moved at random and observed the position of the B-chromosome with respect to the X. Twenty nuclei, either in late metaphase I, anaphase I or metaphase II, in each of the 11 individuals carrying the extra chromosome were analyzed. Using a contingency table followed by a )~2 analysis we found it to segregate at random (Z2= 15.49; 0.20>P>0.10). To test for a possible influence of the supernumerary chromosome on either total or interstitial chiasma frequency, linear regression analysis was carried out only for the Sierra de la Ventana population. In the other two populations less than 5 individuals have the B-chromosome, so the regression analysis was inappropriate. No significant relationship was found between presence of the B-chromosome and either total chiasma frequency (F = 0.88; P > 0.10) or interstitial chiasma frequency (F = 0.97; P > 0.10).
Discussion
Carbonell (unpublished) is of the opinion that representatives of the genus Trimerotropis, coming through the Isthmus of Panama, migrated south along favorable (arid) ecotypes in the Andean chain. They then extended their distribution to the sides of this chain at lower altitudes but only where and when xeric conditions existed, such as the Pacific slope south of Ecuador in Chile and northwestern and Patagonian Argentina in the east. Two South American species of Trimerotropis have been described cytologically: T. ochraceipennis (La Fuente et al., 1968; White, 1973; Mesa, personal communication) and T. pallidipennis (Mesa, 1971 ; Mesa and de Vaio, t976). Both have some metacentric chromosomes, and thus belong to section " B " of the genus (White, 1949). Both species are also polylnorphic. The karyotype of T. pallidipennis was first studied by Coleman (1948) and White (1949) on specimens from different localities in the United States. White (1949) described the species as being structurally homozygous, having three large metacentric autosomal bivalents and a metacentric X chromosome. These same characteristics have been confirmed by Weissman (personal correspondence). Although the sixteen specimens studied by Coleman (1948) were "in a homozygous condition", two of them had only L1 and L2 metacentrics. Neither author observed B-chromosomes in their material. All the structural rearrangements described for the northern populations are also present in the South American populations. Additionally some of the medium size bivalents are highly polymorphic for centromere position and a B-chromosome is present in some individuals of all of the southern populations. Chromosomal polymorphism in the South American populations of T. pallidipennis was first described by Mesa (1971) in five specimens from Puna de Atacama (Argentina) and it was later observed in twelve specimens from different localities of southeastern Argentina (Mesa and Vaio, 1976). We chose to study the chromosomal constitution of fifty individuals, taken from three distant populations in southern Argentina, to see if this inversion system showed geographical variation and whether it influenced chiasma frequency.
382
E.S. de Vaio et al.
Inversion System John and Lewis (1966) have discussed the varying patterns of chromosome polymorphism to be found in insects and their relation to geographic distribution. In the inversion polymorphism of Drosophila it is not uncommon to find that populations in the center of distribution of the species have the highest degree of polymorphism, while marginal populations tend to be less polymorphic. But, even for Drosophila, certain apparently paradoxical situations are known, and this led John and Lewis to conclude that highly polymorphic populations may evolve in any suitable environment, whether this is at the center or at the periphery of the species range. Patterns of chromosome variability have already been studied in two Trimerotropines. Although a clinal pattern of inversion heterozygosity was described by Schroeter (1968) in T. helferi, with high heterozygosity at the center of the species range, a different situation was uncovered by Weissman (1976) in T. pseudofasciata. In this species, populations of high densities in suitable environments, but no necessarily centrally located, are chromosomally polymorphic. On the other hand, populations at low densities, ecologically marginal, but not necessarily peripheral, are chromosomally uniform. It has also been pointed out by White (1973), that some species of Trimerotropis may show clines in the degree of inversion polymorphism but without a central-peripheral pattern. The three populations studied in this paper were found to have significant differences in their inversion systems. Two populations are polymorphic for pericentric inversions while the third is structurally uniform: the Laguna Blanca population, situated in the presumtive pathway of migration of the species toward the South, has the L1, L2 and L3 and X as fixed metacentrics and is structurally monomorphic in the same manner as the northern populations of T. pallidipennis (White, 1949; Weissman, personal correspondence). In the other two populations the three large pairs and the X are also fixed metacentrics but the medium pairs are polymorphic. The Sierra de la Ventana population, located at the absolute eastern border of the species' distribution, is highly polymorphic. The Choele-Choel population, with a typically xeric environment, and situated geographically between the other two, has the highest frequency of inversions with many of them being homozygous.
Chiasma Frequency The three populations differ from each other in total chiasma frequency. Interstitial chiasmata are also greatly reduced in inversion heterozygotes and inversion homozygotes. Laguna Blanca, with only standard homozygotes, has the highest total chiasma frequency, with a great number of interstitial chiasmata. Individuals from Sierra de la Ventana and Choele-Choel have very few interstitial chiasmata (see Table 2). Total chiasma frequency, which is higher in ChoeleChoel than in Sierra de Ia Ventana, is increased by the numerous ring bivalents with two terminal chiasmata in inversion homozygote pairs, According to Zarchi et al. (1972) while interstitial chiasmata represent gen-
Chromosome Polymorphismin Trimerotropispallidipennis
383
uine sites of genetic recombination the function of terminal chiasmata is simply to maintain homologous chromosomes together in meiosis, and thus insure correct segregation. In their view terminal chiasmata have very little relevance to recombination. The inversion system of T. pallidipennis which restricts interstitial chiasmata can therefore be expected to have its consequences on recombination. The reduction of chiasmata in inversion heterozygote bivalents has been described for several species of Trimerotropis (White, 1951 b; White and Morley, 1955; Schroeter, 1968; Weissman, 1976) but disturbed chiasma frequency in inversion homozygote bivalents has only been observed in T. pseudofasciata (Weissman, 1976) and T. pallidipennis (this paper). We agree with Weissman (1976) that recombination in the inversion region of the rearrangement homozygote would have no effect since the gene sequence in both homologous chromosomes are identical. Inversion heterozygosity sometimes results in altered chiasma frequency in the unmodified elements of the complement (Shultz and Redfield, 1951). In four species of Trimerotropis there is an increase in chiasma frequency in regions distal to the rearrangements, so that the total chiasma frequency per cell does not decrease (White and Morley, 1955 ; Schroeter, 1968 ; Weissman, 1976). This effect was not observed in our polymorphic populations. In Sierra de la Ventana chiasmata in the medium chromosomes are terminal and frequently reduced to a minimum of one per bivalent. Perhaps "interference saturation" (White and Morley, 1955) occurs here. The Choele-Choel population has a total chiasma frequency significantly higher than that in Sierra de la Ventana since it is increased by the numerous ring bivalents, with two terminal chiasmata, in inversion homozygote pairs. Whether this can be attributed to a "Schultz-Redfield" compensation effect is not clear. Inversion heterozygosity coupled with the high number of inversions which are present thus impose restrictions on the recombination system in our populations of T. pallidipennis as measured by a reduced number of interstitial chiasmata.
Relationship Between the Inversion System and Chiasma Frequency A definite pattern of chromosome polymorphism and a close relationship of this variability to chiasma frequency has been found in the three southern populations of T. pallidipennis. The population nearest the pathway of migration is structurally uniform and has the highest mean number of interstitial chiasmata. In contrast, the peripheral population of Sierra de la Ventana is highly polymorphic, has very few interstitial chiasmata and the lowest total chiasma frequency. Choele-Choel, geographically intermediate between the other two populations, in a typically xeric environment, has the highest frequency of inversions, with a tendency to homozygosity and, additionally, has a few interstitial chiasmata. The Laguna Blanca population has the same characteristics as the population of T. pseudofaseiata on San Nicolas Island (Weissman, 1976). This island population is also characterized by having the lowest population densities. Laguna
384
E.S. de Vaio et al.
Blanca may represent an ecologically marginal population living in relatively adverse climatic conditions. The peripheral population of Sierra de la Ventana has the highest degree of polymorphism and the lowest total chiasma frequency. It resembles the central populations of T. he(feri (Schroeter, 1968) and the high density populations (in favorable conditions) of T. pseudofasciata (Weissman, 1976) which are structurally polymorphic and have a lower recombination frequency. Probably Sierra de la Ventana, although geographically peripheral, represents a population living in a hospitable environment. Although Schroeter (1968) encountered only one pair of chromosomes in the inversion polymorphism of T. helferi, he found a situation comparable to that of Choele-Choel where high frequencies of inversions result in increases in the number of inversion homozygotes. He attributed this to an increase in the amount of recombination. This is not true in our populations of T. pallidipennis where the high number of inversions impose restrictions on the recombination system as measured by a reduced number of interstitial chiasmata. In this species inversions, in either the heterozygous or homozygous condition, restrict interstitial chiasmata so that no recombination takes place in the inverted segments. This same condition was observed by Weissman (1976) in T. pseudofasciata and led that author to conclude that inversion homozygotes and heterozygotes have similar effects upon chiasma distribution and thus should have similar selective pressures, resulting in a very slow process of fixation of inversions. Inversions would only become fixed in environments which had been stable for a very long period of time (Weissman, 1976). Studies on the ecology and population densities of T. pallidipennis are necessary before we can definitively establish the adaptive significance of its pattern of chromosome variation. In one sense all three populations studied could be considered peripheral if we take North America as the geographic center of the species' distribution. The variation in their chromosome polymorphism may then have arisen as different responses to the need of particular new environments.
B- Chromosome The southern populations of T. pallidipennis are also polymorphic with respect to the presence of a telocentric B-chromosome which is absent in material from U.S.A. (White, 1949; Coleman, 1948). Several B-chromosomes with different structures have been observed in different species of Trimerotropis and, according to Hewitt (1973), they reflect the turbulence in karyotype typical of the rapid evolution which this group is undergoing. The B-chromosome of T. pallidipennis, which is similar to those found in other species of the genus Trimerotropis (White, 1973), has an X-like heterocromatic distal segment coupled with a proximal isopycnotic one. The most extensive work on the adaptive value of B-chromosomes in natural populations has been carried out in the grasshopper Myrmeleotettix maculatus, which is polymorphic for a large metacentric B-chromosome (see Robinson
Chromosome Polymorphism in Trimerotropis pallidipennis
385
and Hewitt, 1976, for references). An increase in chiasma frequency was observed in individuals which posses the B-chromosome (John and Hewitt, 1965a, b) and this increase was found to be different in males and females (Hewitt, 1976). When a possible influence of the B-chromosome of T. pallidipennis on Achromosome chiasma frequency was tested, by linear regression analysis of the Sierra de la Ventana data, no significant effect could be demonstrated. It is possible that chiasma frequency in this population is driven to an extreme lower limit by the inversion system and cannot be raised by the presence of supernumerary chromosome. The number of B-individuals analysed was, however, small and it would cleary be desirable to study larger samples before arriving at a final answer. To test if a preferential segregation of the B-chromosome exists in the spermatogenesis of T. pallidipennis, we studied 20 nuclei of each B-containing individual at stages where the position of the supernumerary chromosome could be observed with respect to the X chromosome. Apparently in this species, the B-chromosome segregates at random with respect to the X chromosome. To describe the exact pattern of variation in the chromosome polymorphism of T. pallidipennis both with respect to A and B-chromosomes it will be necessary to know more about the ecology of the species and to study the chromosomal constitution of populations through the entire species range. We hope to turn to these questions next. Acknowledgements. We wish to thank Dr. Alejo Mesa for suggesting the subject of this paper and providing the material; Dr. B. John and Dr. D. Weissman for their valuable suggestions and the critical reading of the manuscript; Dr. C.S. Carbonell for giving us the systematic and biogeographical data; and Dr. J. Chebataroff for providing the climatic data.
References Carothers, E.E.: The segregation and recombination of homologous chromosomes as found in two genera of Acrididae (Orthoptera). J. Morph. 28, 445-520 (1917) Carothers, E.E.: Genetical behaviour of heteromorphic homologous chromosomes of Circotettix (Orthoptera). J. Morph. 35, 457483 (1921) Carothers, E.E.: The maturation divisions in relation to the segregation of homologous chromosomes. Quart. Rev. Biol. 1, 419435 (1926) Carothers, E.E. : The maturation divisions and segregations of heteromorphic homologous chromosomes in Acrididae (Orthoptera). Biol. Bull. 61, 324 349 (1931) Coleman, L.C. : The cytology of some western species of Trimerotropis (Acrididae). Genetics 33, 519 528 (1948) Evans, W.L.: Cytology of the grasshopper genus Circotettix. Amer. Naturalist 88, 21-23 (1954) Helwig, E.B. : Chromosomal variation correlated with geographical distribution in Circotettix verruculatus (Orthoptera). J. Morph. 47, 1 36 (1929) Helwig, E.B. : Spermatogenesis in hybrids between Circotettix verruculatus and Trimerotropis suffusa (Orthoptera: Oedipodinae). Univ. Colorado Studies Series in Biol. 3, 4-64 (I955) Hewitt, G.M. : The integration of supernumerary chromosomes into the orthopteran genome. Cold. Spr. Harb. Symp. quant. Biol. 38, 183-194 (1973) Hewitt, G.M. : Meiotic drive for B-chromosomes in the primary oocytes of Myrmeleotettix maculatus (Orthoptera: Acrididae). Chromosoma (Berl.) 56, 381-391 (1976) Jackson, R.C.: Chromosomal evolution in Haplopappus gracilis: A centric transposition race. Evolution (Lawrence, Kans.) 27, 243-256 (1973)
386
E.S. de Vaio et al.
John, B., Hewitt, G,M.: The B-chromosome system of Myrmeleotettix maculatus (Thunb.). I. The mechanics. Chromosoma (Bert.) 16, 548-578 (1965) John, B., Hewitt, G.M,: The B-chromosome system of Myrmeteotettix maculatus (Thunb.). II. The statics. Chromosoma (Berl.) 17, t21-138 (1965) John, B., Lewis, K.R. : Chromosome variability and geographical distribution in insects. Science 152, 711-72l (1966) John, B., Weissman, D.B.: Cytogenetic components of reproductive isolation in Trimerotropis thalassica and T. occidentalis. Chromosoma (Bed.) 60, 187303 (1977) King, R.L.: Heteromorphic homologous chromosomes in three species of Pseudotrimerotropis (Orthoptera: Acrididae). J. Morph. 38, 19-63 (1924) La Fuente, N., Esponda, P., Solerivicens, J.: Citotaxonomia en ort6pteros chilenos. Rev. Chil. Ent. 6, 91-99 (1968) Mesa, A.: Polimorfismo cromos6mico en Trimerotropis pallidipennis (Orthoptera: Oedipodinae). Rev. Per. Entom. 14, 229--233 (1971) Mesa, A., Vaio, E.S. de: Polimorfismo cromos6mico en Trimerotropis pallidipennis (Orthoptera: Oedipodinae). Sociedade Brasileira para o Progresso da Ciencia, 28a. Reuniao Anual, Brasilia, Suppl. Ciencia e Cultura Se~ao 4.1.1,282 (1976) Rehn, J.A.G. : The South American species of Oedipodinae genus Trimerotropis (Orthoptera : Acrididae). Trans. Amer. Ent, Soc, 65, 395-414 (1940) Robinson, P.M., Hewitt, G.M. : Annual cycles in the incidence of B-chromosomes in the grasshopper Myrmeleotettix maculatus (Orthoptera: Acrididae). Heredity 36, 399-412 (1976) Schroeter, G.L.: Pericentric inversion polymorphism in Trimerotropis helferi (Orthoptera: Acrididae) and its effects on chiasma frequency. Ph. D. Dissertation Department of Biology-Genetics. University of California, Davis (1968) Shultz, J., Redfield, H.: Interchromosomat effects on crossing over in Drosophila. Cold. Spr. Harb, Syrup. Quant. Biol. 16, 175-197 (1951) White, M.J.D. : Animal cytology and evolution, 1st. edit. Cambridge: University Press 1945 White, M.J.D.: A cytological survey of wild populations of Trimerotropis and Circotettix (Orthoptera, Acrididae). I. The chormosomes of twelve species. Genetics 34, 537--563 (1949) White, M.J.D.: A cytological survey of wild populations of Trimerotropis and Circotettix (Orthoptera, Acrididae). II. Racial differentiation in T. sparsa. Genetics 35, 3153 (1951 a) White, M.J.D. : Structural heterozygosity in natural populations of the grasshopper Trimerotropis sparsa. Evolution (Lancaster, Pa.) 5, 376-394 (1951 b) White, M.J.D. : Cytogenetics of orthopteroid insects. Adv. in Genet. 4, 267-330 (1951 c) White, M.J.D.: Animal cytology and evolution, 3rd. edit. Cambridge: University Press (1973) White, M.J.D., Morley, F.H.W. : Effects of pericentric rearrangements on recombination in grasshopper chromosomes. Genetics 40, 604-619 (1955) Wenrich, D.H. : Synapsis and chromosome organization in Chorthippus (Stenobothrus) curtipennis and Trimerotropis suffusa (Orthoptera). J. Morph. 29, 479-516 (1917) Weissman, D.B. : Geographical variability in the pericentric inversion system of the grasshopper Trimerotropis pseudofasciata. Chromosoma (Berl.) 55, 325-347 (1976) Zarchi, Y., Simchen, G., Hillel, J,, Schaap, T. : Chiasmata and the breeding system in wild populations of diploid wheats, Chromosoma (BerL) 38, 77-94 (1972) Received June 6-September 13, 1978 / Accepted September 13, 1978 by B. John Ready for press November 8, 1978
