Physical variability and fluctuating asymmetry in heterozygous and homozygous populations of <i>Rivulus</i> <i>marmoratus</i>

Abstract

The debate over the influence of heterozygosity and homozygosity on developmental stability is ongoing, and rarely is an animal model found that represents these two genetic states in naturally occurring populations. Rivulus marmoratus (Pisces: Aplocheilidae), a small cyprinodontid fish indigenous to mangrove marshes of the western tropical Atlantic coast, may be an appropriate model. This species is well known as the only self-fertilizing hermaphroditic vertebrate, and populations normally consist of genetically diverse groups of homozygous clones. However, male fish are known from a few populations, and outcrossing has been documented from one area (Belize in Central America) where heterozygosity is the norm. Are there differences in developmental stability, as represented by fluctuating asymmetry (FA) and physical variability, between naturally occurring homozygous and heterozygous populations? Thirty-one meristic and morphometric characters in 180 fish from 10 populations were examined to determine physical variability. In addition, FA and the presence of physical anomalies were analyzed. Belize fish (heterozygous) were compared with eight other (homozygous) populations. To control for possible environmental effects, offspring of two populations (Belize and Florida) were reared at 25°C and similarly analyzed. The FA analyses and variability comparisons revealed no differences between heterozygous and homozygous populations, and the same was true for fish reared at a constant temperature. However, some populations (e.g., Brazil) were more variable than others. These results indicate that coadapted gene complexes may be more important than heterozygosity in canalizing developmental stability in this animal.Résumé : La controverse au sujet de l'influence de l'hétérozygotie et de l'homozygotie sur la stabilité du développe-ment se poursuit et il est difficile de trouver un modèle animal qui puisse représenter les deux conditions génétiques chez des populations naturelles. Rivulus marmoratus (Pisces : Aplocheilidae), un petit poisson cyprinodonte indigène des mangroves de l'Atlantique tropical occidental, est potentiellement un bon modèle. C'est la seule espèce de vertébré hermaphrodite à s'auto-féconder et les populations sont ordinairement constituées de groupes génétiquement diversifiés de clones homozygotes. Cependant, les mâles existent chez quelques populations et des croisements allogames ont pu être réalisés en une région (Bélize, l'Amerique centrale) où l'hétérozygotie est la condition normale. Existe-t-il des différences dans la stabilité du développement, telles que mesurées par l'asymétrie fluctuante et la variabilité physique, entre les populations naturelles homozygotes et hétérozygotes? Trente et une mesures méristiques et morphométriques relevées sur 180 poissons de 10 populations ont permis d'évaluer la variabilité physique. En outre, l'asymétrie fluctuante et la présence d'anomalies physiques ont été analysées. Les poissons du Bélize (hétérozygotes) ont été comparés à ceux de huit autres populati...

Androdioecy in Kryptolebias killifish and the evolution of self-fertilizing hermaphroditism

“…The absence of male specimens in all known collections of K. ocellatus taken together with the results of the histological examination of gonads indicate that this species is composed of simultaneously functional hermaphroditic individuals. This is similar to that recorded for K. marmoratus, in which over 250 specimens collected in Florida were all hermaphrodites (Taylor, 1990) and not more than 15 wild males were reported to be collected over several decades of study (Taylor, 2001). In addition, live isolated individuals of K. ocellatus produced fertilized eggs in the laboratory, demonstrating that it is a self-fertilizing species, a condition previously confirmed only for K. marmoratus.…”

Heterozygosity and asymmetry: Ectodysplasin as a form of genetic stress in marine threespine stickleback

“…Negative correlations between asymmetry and genomic heterozygosity would provide evidence for such a mechanism, but we found no such association. This observation contrasts with many studies that reported lower FA in relatively heterozygous individuals (Leary et al 1983;Roldan et al 1998;Vøllestad et al 1999), but is consistent with findings for European stickleback (Van Dongen et al 2009;Trokovic et al 2012, see also Taylor 2001). Stickleback plate asymmetry is heritable (h 2 = 0.21, Loehr et al 2012) and reduces fitness (Reimchen and Nosil 2001;Bergstrom and Reimchen 2003, but see Bergstrom and Reimchen 2005;Reimchen and Bergstrom 2009), yet was not associated with decreased genomic heterozygosity.…”

Temporal variation in the pollen:ovule ratios of Clarkia (Onagraceae) taxa with contrasting mating systems: field populations

“…This reasoning assumes that selfers exhibit the same level of intrinsic developmental stability as outcrossers. If, however, highly selfing genotypes exhibit greater developmental instability due to reduced homeostasis (Lerner, 1954;Kramer et al, 2002;Hall, 2005;Pedersen et al, 2005;Pertoldi et al, 2006; but see Taylor, 2001), then any selection favouring a constant P:O in selfers might be constrained by increased developmental instability. This prediction also assumes that temporal variation in mating system and mating opportunities experienced by congeners is sufficiently large to select for different patterns of sex allocation in these taxa.…”

“…, 2005; Pertoldi et al. , 2006; but see Taylor, 2001), then any selection favouring a constant P:O in selfers might be constrained by increased developmental instability. This prediction also assumes that temporal variation in mating system and mating opportunities experienced by congeners is sufficiently large to select for different patterns of sex allocation in these taxa.…”