Geographical variation in the male intromittent organ of the Trinidadian guppy (Poecilia reticulata)

Abstract

Sexual selection may favour the evolution of elaborated genital traits in males, particularly when phenotypic variation in such traits results in corresponding variation in reproductive success among males in the population. Compared with insects, very little is known about the natural variation in any male genital trait, and its causes, in vertebrates. Here we report on variation in a male intromittent organ both within and between natural populations of a vertebrate, the Trinidadian guppy (Poecilia reticulata). Male guppies inseminate females using an intromittent organ called the gonopodium. We demonstrate that males from populations that have evolved under high fish-predation intensity have, on average, a relatively longer gonopodium than males originating from populations under low fish-predation intensity. Compared with body coloration, the gonopodium exhibited relatively low phenotypic variation, but nonetheless was within the range of known variation for sexually selected traits. The male gonopodium was positively allometric in general. To our knowledge, this is the first report of within-species variation in an intromittent organ and of a positive allometric relationship between male genitalia and body size in a vertebrate species. Our results suggest that the length of the male intromittent organ in the guppy is under selection, which varies geographically.

Male genital size reflects a tradeoff between attracting mates and avoiding predators in two live-bearing fish species

“…Interestingly, the pattern observed in the two Gambusia species in the present study is opposite that observed in previous studies of intraspecific divergence in gonopodium size (9,54). In these other studies, males exhibited larger gonopodia in predatory environments compared with low predation localities, indicating that predation's influence on gonopodium diversification may be complicated.…”

Sexual selection and genital evolution

“…Although this study was criticized on methodological grounds [44], re-analysis did little to change the initial conclusions [45]. Negative genital allometry has subsequently been reported in other species [46], but there are cases of positive genital allometry [47,48]. Nevertheless, if the intromittent organ is generally under stabilizing selection, as appears to be the case [43], it raises the interesting question of how divergence is initiated.…”

Genital morphology and fertilization success in the dung beetleOnthophagus taurus: an example of sexually selected male genitalia

“…The average coefficients of phenotypic variation were 5.9% across seven genital traits in the waterstrider G. incognitus (Arnqvist & Thornhill 1998), 8.1% for the length of the gonopodium of Poecilia reticulata (Kelly et al 2000), 10.8% for the length of the gonopodium of Brachyrhaphis episcopi ( Jennions & Kelly 2002) and a mean of 6.5% for a variety of genital traits measured across 20 species of insect and spider (Eberhard et al 1998). Genital traits were at least as variable as general body size, a finding also consistent with Arnqvist & Thornhill's (1998) study of waterstriders.…”