doi: 10.1111/j.1420-9101.2004.00778.x
Potential fitness benefits from mate selection in the Atlantic cod (Gadus morhua) G. RUDOLFSEN,* L. FIGENSCHOU,* I. FOLSTAD,* J. T. NORDEIDE & E. SØRENG *Department of Evolution and Ecology, Institute of Biology, University of Tromsø, Tromsø, Norway Department of Fisheries and Natural Sciences, Bodø Regional University, Bodø, Norway
Keywords:
Abstract
breeding ornaments; cod; fitness benefits; Gadus morhua; milt quality; sexual selection.
Little evidence of benefits from female mate choice has been found when males provide no parental care or resources. Yet, good genes models of sexual selection suggest that elaborated male sexual characters are reliable signals of mate quality and that the offspring of males with elaborate sexual ornaments will perform better than those of males with less elaborate ornaments. We used cod (Gadus morhua L.), an externally fertilizing species where males provide nothing but sperm, to examine the potential of optimal mate selection with respect to offspring survival. By applying in vitro fertilizations, we crossed eight females with nine males in all possible combinations and reared each of the 72 sib groups. We found that offspring survival was dependent on which female was mated with which male and that optimal mate selection has the potential to increase mean offspring survival from 31.9 to 55.6% (a 74% increase). However, the size of the male sexual ornaments and sperm quality (i.e. sperm velocity and sperm density) could not predict offspring survival. Thus, even if there may be large fitness benefits of mate selection, we might not yet have identified the male characteristics generating high offspring survival.
Introduction Sexually selected characters in males are assumed to reveal information about individual qualities (Hamilton & Zuk, 1982; Andersson & Iwasa, 1996). Consequently, nonrandom mating and expression of female preference are commonly observed when males provide parental care or other resources for the female (Milinski & Bakker, 1990, reviewed in Andersson, 1994; Iyengar & Eisner, 1999). Further, females could potentially allocate egg numbers, egg quality or parental care according to perceived mate quality (Reynolds & Gross, 1992; Norris, 1993; Petrie & Williams, 1993; Moore, 1994; Møller, 1994; Petrie, 1994; Von Schantz et al., 1994; Hasselquist et al., 1996; Jia & Greenfield, 1997; reviewed in Sheldon et al., 1997; Alatalo et al., 1998; Hoikkala et al., 1998; Watson, 1998; Wedekind et al., 1998; Gil et al., 1999; Correspondence: Geir Rudolfsen, Department of Evolution and Ecology, Institute of Biology, University of Tromsø, N-9037 Tromsø, Norway. Tel.: +47 77 64 63 57; fax: +47 77 64 63 33; e-mail:
[email protected]
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Cunningham & Russell, 2000, 2001). However, less evidence of fitness benefits from mate choice has been found in species were such potential confounding factors are eliminated (Andersson, 1994; Sheldon, 2000). Recent studies that experimentally control for nongenetic benefits provide empirical evidence for a genetic basis of offspring viability (Welch et al., 1998; Wedekind et al., 2001; Welch, 2003). Moreover, variation in male characters known to be preferred by females in general explains only 1.5% of the variance in offspring viability (Møller & Alatalo, 1999). However, in the alpine whitefish (Coregonus sp.) where males provide nothing but sperm, variation in the expression of male sexual ornamentation could explain up to 32% of the variance in offspring survival (Wedekind et al., 2001). Additionally, average offspring survival differed with more than 10% depending on which male the females were crossed with (Wedekind et al., 2001). Mate choice may therefore have a large effect on reproductive success, even when no parental care is provided. An alternative hypothesis to mate choice for superior genes is mate choice for genetic compatibility (Tregenza
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Potential fitness benefits and mate selection
& Wedell, 2000; Zeh & Zeh, 2001) and for example polyandry has been suggested to enhance female reproductive success by reducing genetic incompatibility in offspring (reviewed in Jennions & Petrie, 2000 and Tregenza & Wedell, 2000). Offspring viability would in this scenario be related to particular combinations of genotypes and consequently the most suitable male for one female may not be the best for another. Two recent studies have shown that genes within the major histocompatibility complex (MHC) might influence mate compatibility. Atlantic salmon (Salmo salar) choose their mates in order to increase offspring MHC heterozygosity and thereby provide them with better defense against parasites and pathogens (Landry et al., 2001). Moreover, relationships through mate choice have been found in female sticklebacks (Gasterosteus aculeatus), which try to achieve an optimum number of MHC alleles for their offspring (Aeschlimann et al., 2003). The Atlantic cod (Gadus morhua L.) aggregate at spawning areas in spring and their mating behaviour resembles that of other lekking species (Nordeide & Folstad, 2000). In the last decades, dramatic population declines of cod of up to 99% have been reported (Hutchings, 2003 in Rowe & Hutchings, 2003). As the cod is most heavily exploited during the annual mating aggregations, male skewed catches, disturbance of male hierarchies and selective removal of dominant males might negatively affect the availability of high quality males for females (Morgan & Trippel, 1996; Nordeide, 1998; Rowe & Hutchings, 2003). This may in turn disturb or hinder mate choice and reduce female reproductive success, resulting in reduced recruitment and lowered population size (Salvanes & Balino, 1998; Reynolds & Jennings, 2000; Rowe & Hutchings, 2003). However, effects because of mate choice disruptions are hard to quantify and have so far not been incorporated in population models (Rowe & Hutchings, 2003). Cod spawn repeatedly and each female produces millions of small eggs that are externally fertilized; no parental care is provided (Kjesbu, 1989; Kjesbu et al., 1991, 1996). Although the natural mating behaviour of cod is not well known, male–male competition, male display involving both auditory and visual signals, and female mate selection are observed in experimental studies (Brawn, 1961; Hutchings et al., 1999; Bekkevold et al., 2002). The males’ acoustic signals, used during display behaviour, are produced by contractions of paired, striated drumming muscles surrounding two to four external lobes of the swimming bladder wall (Brawn, 1961; Hawkins, 1993; own observations). Males have larger drumming muscles than females and the sound they produce is suggested to be important in mate assessment (Engen & Folstad, 1999; Hutchings et al., 1999; Nordeide & Kjellsby, 1999). Drumming muscle size is also related to male fertilization potential, measured as sperm density (Engen & Folstad, 1999). Before engaging in spawning, males display their fin size by lowering and
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erecting both the dorsal and ventral fins during a circling courtship dance (Brawn, 1961). When milt and eggs are released, males swim up-side down beneath the females in a ‘ventral mount’ position. Here the male has to match the females’ swimming speed while grasping her with his pelvic fins (Brawn, 1961; Hutchings et al., 1999; Rakitin et al., 2001). Thus, the size of fins and fin muscles may be of importance to ensure that the urogenital openings of both fish are aligned opposite one another during the simultaneous release of milt and eggs. Consequently, the courtship behaviour may be indicative of fertilization potential and thus be of importance for males’ reproductive success (Hutchings et al., 1999). Spawning pairs are often joined by several satellite males that shed their sperm among the newly released eggs (Brawn, 1961) and, when mating in tanks, eggs from a single bout are often fertilized by more than one male (Hutchings et al., 1999; Rakitin et al., 2001; Bekkevold et al., 2002). Although sperm competition is only documented in captivity, large ejaculate size (Stockley et al., 1997) and the up to 1 h longevity of both sperm and unfertilized eggs (Trippel & Morgan, 1994) suggest that sperm competition also occurs under natural conditions. As sperm density and sperm velocity could predict male reproductive success (Froman & Feltmann, 1998; Birkhead et al., 1999; Rakitin et al., 1999a; Gage et al., 2002, 2004), these ejaculate characteristics are likely to be under strong sexual selection. In our experiment, we fertilized eggs in vitro and females could thus not bias reproductive investment in response to the quality of the males. By cross-fertilizing all possible parental combination we were able to examine the potential benefits of mate selection and to relate male sex traits (i.e. both primary and secondary sex traits) to offspring survival. According to the good gene hypotheses, we predicted that males with large secondary sexual traits (i.e. drumming muscle mass, fin size and fin muscles mass) and high milt quality (i.e. high sperm velocity and high sperm density) should be males of high quality and consequently sire offspring with enhanced survival.
Materials and methods Parental fish and gamete collection On 3 April 2003, sexually mature adult Atlantic cod were randomly selected from trawl captures at Henningsværstrømmen, (6811¢N, 147¢E), Northern Norway. The fish were kept in outdoor tanks with flowing seawater until handling. Ten ripe males and 10 ripe females were initially selected for the breeding experiment. We collected their gametes in separate plastic beakers by gently pressing the abdomen after drying the fish surface to avoid contaminating the samples. On the basis of their otoliths and synaptophysin (Syp I) locus, as respectively described by Rollefsen (1934) and Fevolden & Pogson (1997), nine of the males and eight of the
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females were identified as belonging to the north-east Arctic population of Atlantic cod. The three remaining parental fish belonged to the coastal population and their offspring were not included in later analyses. The gametes were stored at approximately seawater temperature until video recordings of sperm motility and fertilization. To be able to estimate the effect of gamete storing on offspring survival, time from stripping until fertilization was recorded. The effect of storage time on survival of both eggs (334–573 min) and milt (78– 400 min) was evaluated with a multiple regression (type I sums of square). Storage time of eggs was found to have a significant effect on offspring survival (F1,141¼ 28.83, P < 0.001). Storage time of milt on the contrary, did not have any significant effect on offspring survival, neither alone (F1,141 ¼ 0.01, n.s.) nor after removing the effect of egg storage time (F1,141 ¼ 0.25, n.s.). Storage time of eggs was therefore included as a covariate in the later analysis. Fertilization and breeding As fertilization success depends on sperm density (Rakitin et al., 1999a), milt volume used in fertilization was adjusted to sperm density so that approximately the same number of sperm cells was used in all 72 parental combinations. The milt samples used for fertilization varied from 0.37 to 0.86 mL (volume adjusted for sperm density) and were diluted in 50 mL seawater immediately before each fertilization. This sperm : seawater dilution should, according to Trippel & Neilson (1992) give more than 95% fertilization success. For each sib group, the sperm : seawater solutions were poured over 2 mL of eggs in ovarian fluid, and the fertilized eggs were immediately rinsed with seawater to avoid polyspermy (Ginzburg, 1972; Styan & Butler, 2000; Yund, 2000; Franke et al., 2002). The freshly fertilized eggs were then transported to the hatchery in bottles filled with unfiltered seawater at seawater temperature (approximately 6 C). In the hatchery each sib group was divided over two beakers that were positioned in different tanks. Each beaker was randomly positioned in each tank, giving two replicates of each sib group (mean egg number in each beaker ¼ 244, SD ¼ 59.53). Prior to the survival analyses, we examined if there were effects of experimental setup. There was no difference in mean egg survival between the two replicates sired in the two different tanks (paired t-test, t71 ¼ 0.103, n.s.). Further, we examined the effect of beaker position by entering the beaker position from both axis of the tank as a fixed factors, with offspring survival as the depend variable in an A N O V A . There was no significant effect of neither the axis, nor any significant interaction (F10,47 ¼ 0.5980, n.s., F10,47 ¼ 0.5109, n.s., F76,47 ¼ 0.8745, n.s., respectively). Seawater, taken from 50 m depth and maintained at approximately 6 C, was continuously flowing through each beaker and was not rinsed or filtered. During the 19 days long hatching period, we checked the beakers at
day 3, 5, 8, 10 and 12 to count and remove possible unfertilized, dead or misdeveloped eggs, eyelings or fry. Mortality of these eggs, eyeling and fry were immediately ascertained under a microscope. Survival was estimated as the number of eyelings or fry alive at day 19 over the initial number of eggs. At day 19 the larvae were still feeding on their egg yolks and mortality because of starvation had not yet occurred (Yin & Blaxter, 1986). Eyelings and fry were killed with benzocain. Ejaculate characteristics Sperm motility analyses were conducted by using an aliquot (
