EVOLUTION & DEVELOPMENT
11:3, 318 –322 (2009)
DOI: 10.1111/j.1525-142X.2009.00334.x
Saltational evolution of trunk segment number in centipedes Alessandro Minelli,a, Amazonas Chagas-Ju´nior,b and Gregory D. Edgecombec a
Dipartimento di Biologia, Universita` degli Studi di Padova, Via Ugo Bassi 58 B, I-35131 Padova, Italy Laborato´rio de Aracnologia, Departamento de Invertebrados, Museu Nacional, UFRJ, Quinta da Boa Vista, s/no, Sa˜o Cristo´va˜o, CEP-20940-040, Rio de Janeiro, RJ, Brazil c Department of Palaeontology, Natural History Museum, Cromwell Road, London SW7 5BD, UK b
Author for correspondence (email:
[email protected])
SUMMARY Saltational changes in segment numbers have likely occurred in arthropod evolution, especially if mechanisms of segment formation involve a multiplicative phase, as recently suggested in the evo-devo literature. Here we provide for the first time evidence of major phenotypic saltation
in the evolution of segment number in a lineage of centipedes, with a newly discovered species of scolopender having segment numbers duplicated with respect to its closest relatives, and to all the remaining 7001 species of Scolopendromorpha known to date.
INTRODUCTION
Here we provide evidence of major phenotypic saltation in the evolution of segment number in centipedes, most conspicuously in a newly discovered species of scolopender, in which segment numbers are approximately twice as many as in all other Scolopendromorpha, including the species’ closest relatives. Comparative morphology excludes the likelihood for intermediates to have existed and phylogenetic reconstruction argues strongly in favor of a recent occurrence of this dramatic morphological change.
The status of evolutionary developmental biology (evo-devo) with respect to the Neo-Darwinian paradigm of evolution is widely debated (e.g., Wagner 2001; Arthur 2002; Laubichler and Maienschein 2007; Mu¨ller 2008). Opinion varies from considering evo-devo as fundamentally compatible with NeoDarwinism, although arguably complementing it with respect to its power to explore the inequalities in the landscape of possible forms into which a species can evolve (Hendrikse et al. 2007), to regarding evo-devo as a radical alternative to the traditional views. However, to simply contrast evo-devo with Neo-Darwinism will arguably lead nowhere, as either approach is a complex of theories that must be confronted individually. One of these questions is continuity versus discontinuity in phenotypic evolution. In this respect, the profound phenotypic effects of point mutations in genes of major developmental relevance have suggested the possibility of saltational change, to the extent of reviving Goldschmidt’s (1940) long damned concept of ‘‘hopeful monster’’ (e.g., Whiting and Wheeler 1994; Theien 2006). Arthropod segmentation has been suggested to involve, to some extent at least, a multiplicative phase of segment formation (Minelli and Bortoletto 1988; Minelli 2000, 2001). A critical implication of this model is that evolutionary changes in segment-producing mechanisms are likely to generate discontinuous distributions of segment numbers. In particular, an extra segment duplication occurring at the end of an ancestor’s segmentation schedule would give rise to a descendant with a more or less duplicated number of segments. At the phenotypic level, this would arguably qualify as a saltation. 318
ARTHROPOD SEGMENTATION Mechanisms of segmentation and their evolution are a popular issue in evo-devo. A major contrast has emerged between interpretations favoring either a single origin of segmentation at the root of the bilaterian animals or multiple independent origins in annelids, arthropods, and vertebrates. Irrespective of the phylogenetic scenario, nearly all models of segmentation take for granted that segments are specified in anteroposterior progression (Chipman and Akam 2008). However, as mentioned before, segmentation is not necessarily sequential, or fully sequential, but can instead have a hierarchical, or multiplicative component. This means that a first, generally sequential phase will produce a small number of primary segments, each of which is subsequently split, once or multiple times, eventually giving rise to the whole series of body segments. This model was proposed by Maynard Smith (1960) to account for the unusual constancy of segment number in many myriapod species where no intraspecific variation is found for this character. & 2009 Wiley Periodicals, Inc.
Minelli et al. Indications from comparative morphology and phylogenetics for a multiplicative mechanism of segmentation in myriapods, or in arthropods more generally, have thus far found limited support from developmental genetic studies. The scenario, however, has been changing. Whereas expression stripes for the segment polarity gene engrailed are consistent with a strict antero-posterior sequence of segment formation (Kettle et al. 2003), a double segment periodicity, comparable to the primary expression of pair-rule genes in Drosophila, is now known to be involved in segment generation in the geophilomorph centipede Strigamia maritima (Chipman et al. 2004). In several arthropods, each trunk segment is subdivided into an anterior and a posterior component, which often behave, in morphogenesis, as two independent, segment-like units. This is elegantly shown by those naturally occurring ‘‘monsters’’ where the left and right halves of one of the two subunits (pretergite and metatergite) of the dorsal exoskeleton of a segment are wrongly coupled with the corresponding sclerites of the other body side (examples from the geophilomorph centipede Stigmatogaster subterranea are described by M. Lesniewska et al., unpublished data). To this widespread pattern of segmental duplication, some arthropod clades add further, quite compelling evidence in favor of a multiplicative mechanism of segmentation. Centipedes, in particular, provide a body of evidence in support of segmentation more likely being multiplicative rather than sequential, especially from the regularities in trunk segment number separated by major discontinuities (Minelli and Bortoletto 1988).
EVOLUTION OF CENTIPEDE SEGMENT NUMBER Of the five major lineages in this arthropodan group, the number of leg-bearing segments is fixed in the adults at 15 in the Scutigeromorpha, Lithobiomorpha, and Craterostigmomorpha, and this is strongly supported as being the primitive number in centipedes (Edgecombe 2007; Edgecombe and Giribet 2007) (Fig. 1). A majority of species in the Scolopendromorpha have 21 leg-bearing segments and the remainder have 23. Finally, the number of leg pairs in the Geophilomorpha ranges between 27 and 191, but limited to odd numbers in this interval. The fact that the postembryonic development of lithobiomorphs includes instars with an even number of leg pairs is irrelevant to the issue of whether an early specification of odd numbers is a generality for centipedes. It would be unwarranted to equate segment number in those juveniles with the number of fully developed leg pairs they possess. Through the course of early postembryonic stages, the total number of body ‘‘segments’’ can be determined only in an arbitrary manner, because of the frequent mismatch between
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the number of tergites, sternites, leg pairs (only the fully developed ones, or the total number, including those still stumplike which will be articulated only following one more molt). We could add the serially repeated internal organs, e.g., the ventral ganglia. In Lithobius, it has been recently shown, for example, that the number of recognizable neuromeres is generally larger than the number of leg pairs and even increases during one intermolt period (Minelli et al. 2006). Structures ‘‘frozen’’ by their cuticular envelope, such as tergal and sternal plates and leg pairs, cannot change in number in such a continuous way, but ‘‘soft’’ organs behave otherwise. The obvious postembryonic growth of the posterior (subterminal) part of the trunk in these (anamorphic) centipedes is thus compatible with the hypothesis that their full segment complement is already specified, even if not fully realized, before hatching. At variance with the overall conservatism of segment number in large and morphologically diversified lineages such as malacostracan crustaceans and insects, centipede evolution has thus been accompanied by conspicuous transitions in segment number. First is an increase from 15 trunk segments to 21 or 23, apparently without any intermediate, followed by an increase toward the higher or much higher numbers found in geophilomorphs. This latter transition was likely more impressive than present segment number distribution would suggest, as the lowest numbers found in a few geophilomorph species (e.g., 27 or 29) are very probably the result of a secondary reduction. Most geophilomorphs exhibit intraspecific variation and differences between the sexes in segment number. Only in one geophilomorph family (Mecistocephalidae) is segment number generally fixed within the species and identical between the sexes. Whenever sexual differences occur, females have more segments on average than the conspecific males. Two conspicuous features are observed in the segment number distribution among centipedes. First, no single specimen has been recorded in any centipede species with an even number of adult leg-bearing segments apart from a male specimen of the geophilomorph S. maritima with a pair of genital appendages (gonopods) homeotically transformed into a pair of extra legs (Kettle et al. 1999), and another male geophilomorph specimen, belonging to a Polish population of S. subterranea, from which a very large number of ‘‘segmental monsters’’ has been reported (M. Lesniewska et al., unpublished data). Secondly, there are species without intraspecific variation in segment number with up to 53 leg-bearing segments, a numerical precision not easily produced by sequential segmentation of a posterior, subterminal generative zone, but beyond that number numerical stability is liable to break down. Within the Mecistocephalidae, a large majority of species has invariant segment number, though a few species of Mecistocephalus have limited (N and N12) variability (Uliana et al. 2007). Intraspecific variation increases dramatically in
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Fig. 1. Segment number in select centipede clades and putative changes in segment numbers along the main branches. Segment number is constant (N 5 15) in Scutigeromorpha, Lithobiomorpha, and Craterostigmomorpha. Diversity of segment number within the Geophilomorpha ( 5 Mecistocephalidae1Adesmata; N 5 27–191) is not explored here in detail, whereas all putative changes in segment number within the Scolopendromorpha (N 5 21, 23, 39, 43) are specified. The tree is a simplified version of the phylogenetic relationships reconstructed in Edgecombe and Koch (2008) and Koch et al. (2009).
the most segment-rich species in the family, M. microporus, of which specimens with 93, 95, 97, 101 leg-bearing segments are known (Bonato et al. 2001). This enhanced amount of variation in species with a number of segments substantially higher than in their closest relatives is compatible with a recent change in a multiplicative phase of segmentation.
segments less than exactly twice the total number in its ancestor (Minelli 2000). An additional feature most strictly arguing in favor of an origin of this centipede’s segmental organization by wholesale duplication of a pre-existing set is the fact that specimens have either 39 or 43 pairs of legs, two classes four units apart,
PHYLOGENETIC EVIDENCE FOR SALTATIONAL DUPLICATION IN SEGMENT NUMBER We report here on a newly discovered example of a saltational change in segment number, in striking agreement with a simple change in the hypothesized multiplicative phase of segmentation. This example is provided by Scolopendropsis duplicata (Fig. 2A), a recently described species of Scolopendromorpha from Brazil (Chagas-Ju´nior et al. 2008). In having either 39 or 43 pairs of trunk legs rather than 21 or 23, this centipede is so dramatically different from all other scolopendromorph species described to date that it required modifying the current diagnosis of the order. In all other respects, however, it agrees so closely with other taxa from the same geographic region that it can be easily accommodated within an existing genus, with whose known species it shares numerous complex morphological details (Chagas-Ju´nior et al. 2008). The extraordinary feature of this centipede is the number of its leg-bearing segments, which is nearly double with respect to all other scolopendromorphs, and also variable intraspecifically. In comparing a species with 21 or 23 segments with one with 39 or 43 segments, we must allow that the extra segment ‘‘doubling’’ will unlikely produce a leap from N to exactly 2N. Similar to the secondary subdivision into annuli (mostly three or five) of each body segment in leeches, secondary segmentation in arthropods is arguably uniform throughout the body, but incomplete at either end of the series, thus causing the duplicated form to have a couple of
Fig. 2. Scolopendropsis duplicata (A, C; paratype IBSP 2392) and S. bahiensis (B, D; specimen MNRJ 15325): habitus (A, B; scale bar 5 5 mm) and ventral view of the sternum and the flanking coxopleura of the last leg-bearing segment(C, D; scale bar 5 1 mm).
Minelli et al. precisely as expected from segment duplication starting from a species with variability limited to two classes N and N12. For N 5 21, this is exactly the interspecific variation found in the most closely related species, Scolopendropsis bahiensis, recently shown to be the only scolopendromorph centipede with intraspecific variability in segment number (Schileyko 2006), before the discovery of S. duplicata. Preliminary evidence suggests that S. duplicata is perhaps sexually dimorphic in respect to the number of trunk segments, with specimens with 39 pairs of legs being male and those with 43 female, but at the moment we are unsure whether this is actually fixed in the species. A possible objection against segment duplication playing a role in the origin of the sudden increase in segment number in S. duplicata is the segmental distribution of the spiracles in the new species. This argument requires a brief description of spiracle distribution in the other Scolopendromorpha. Apart from one species (Plutonium zwierleini) in which a pair of spiracles is present as an uninterrupted series throughout the trunk, in the other scolopendromorphs there is a fairly regular alternation between segments provided with spiracles and segments without. As a rule, spiracle-bearing segments have perceptibly longer tergites than those lacking spiracles. This regular arrangement is disrupted shortly behind the first third of trunk length, where two consecutive leg-bearing segments (the 7th and the 8th) have long tergites. Thus, in scolopendromorphs with 21 pairs of legs spiracles are found on legbearing segments 3, 5, 8, 10, 12, 14, 16, 18, and 20, sometimes with an additional pair on segment 7; in the species with 23 pairs of legs, there is an extra pair of spiracles on segment 22. What then in S. duplicata? One could expect that the ‘‘segmental anomaly’’ usually found between segments 7 and 8 should be displaced to about segments 13 to 16, if all segments of a typical scolopendromorph are actually duplicated in this species. But this is not the case. In S. duplicata, spiracles occur on segments 3, 5, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, and 38 in the specimens with 39 pairs of legs and also on segments 40 and 42 in the specimens with 43 pairs. This conserved distribution would appear to be more in agreement with a hypothesis of segment increase by posterior addition rather than with the duplication hypothesized here. However, the expected displacement of the ‘‘segmental anomaly’’ from segments 7–8 to about segments 13–16 would only occur if the positioning of a molecular segmental marker eventually translating into the overt segmental disruption would occur, in development, before the duplication of segments. In contrast, if this marker is positioned after the completion of local segmentation and if its position is specified in terms of its distance (measured as number of segments) from the body’s front end, we shall expect positional conservation, as observed in Scolopendropsis, despite the increase in total segment number. As a matter of fact, we do not have any evidence in favor of either temporal sequence (duplication
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first, or positional marker first). We can only observe that laying down the molecular marker on which the future overt patterning will depend at exactly the moment of segmentation is something we know in vertebrates, but arthropods seem to work otherwise. The relative timing of expression of pair-rule and segment polarity genes with respect to Hox genes in Drosophila (e.g., Carroll et al. 2005) is an example. Segmental duplication in Scolopendropsis is evolutionarily recent, the genus being firmly nested within several monophyletic subgroups of the Scolopendromorpha (Edgecombe and Koch 2008; Koch et al. 2009) (Fig. 1). The sister species pair, S. bahiensis and S. duplicata, are morphologically similar apart from the striking difference in trunk segment number (Fig. 2, A and B). This similarity extends to very subtle morphological detail of taxonomical importance such as the coxopleuron, including its pore field and spinulation (Fig. 2, C and D). An additional argument in favor of an extremely strict relatedness of the two species is provided by their geographic patterns (S. duplicata being found at the periphery of the range of the more widespread S. bahiensis: see ChagasJu´nior et al. 2008: fig. 15 for distribution map), suggestive of S. duplicata being a peripheral isolate of S. bahiensis. These indications for the evolutionary recency of the segmental duplication in Scolopendropsis show that transspecific evolution of segment number is not necessarily the effect of selection or genetic drift acting on a principally continuous intraspecific variation (Vedel et al. 2008), but can instead have the character of a phenotypic saltation. It was arguably easy, mechanistically, to extend the scope of a previously existing developmental mechanism of segment duplication, but the consequences on the phenotype have been dramatic indeed. This discovery invites a revisitation of two other likely cases of sudden segment duplication in centipedes, both of them concerning the Geophilomorpha, the lineage representing the sister group of the Scolopendromorpha. One case is found in the large genus Mecistocephalus, within which cladistic analysis identified M. multidentatus, with 49 pairs of legs, as the likely sister species of the 93–101 segmented M. microporus (Bonato et al. 2003). The other case is provided by Orphnaeus heteropodus, a geophilomorph from Mozambique, whose females were reported to possess twice as many trunk segments as the conspecific males (Lawrence 1963).
DISCUSSION The discovery of a scolopendromorph species with recently duplicated segment number is relevant to the changes in segment number associated with the origin of the Geophilomorpha. The geophilomorphs with the lowest segment numbers belong to arguably ‘‘modern’’ families, Geophilidae and Schendylidae, and also represent highly derived clades within those families, and thus are most likely
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derived from ancestors with higher segment numbers. In the most basal representatives of the most basal geophilomorph clade, the Mecistocephalidae, segment numbers are mostly 41 or 43 (Bonato et al. 2003). Thus, the primitive segment number for geophilomorphs is likely 41 or 43, that is, a value virtually or precisely identical to the number in S. duplicata. As in the case of the latter species, the range of numbers we infer as basal for Geophilomorpha was very likely obtained in a saltatory way starting from 21 or 23 (the typical segment numbers in the sister group, Scolopendromorpha, one of which can parsimoniously be inferred to be the general condition for the scolopendromorph–geophilomorph clade, Epimorpha, as a whole). If so, this would suggest that identical, or virtually identical leaps in segment numbers may have occurred multiple times in the course of centipede history, something made possible by the simplicity of the mechanism (wholesale duplication) involved in these events.
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