Many ASACs ago, Dr. Brian Langerhans became a veritable cult hero on Abaco with his presentation on mate choice by female mosquitofish (see posts here and here for detail). In short, they clearly prefer males that have a longer gonopodium – the mosquitofish equivalent of a penis. His recent research takes this further – shape of the gonopodium varies too (insert joke of your choice here). The finding is presented in this paper, one of a wave of recent and upcoming papers from the Langerhans lab (search “Langerhans” in the paper archive for more).
The penis, well gonopodium, tips are shown above (focus on the two photographs on the right). What is first apparent is the complex morphology at the gonopodium tips, something not obvious to the naked eye. Likewise to a female mosquitofish, these structures are not conspicuous, and thus are unlikely to influence her mate choice (may be a good thing, as those hooks look painful). Instead, these complex tips may be involved with sexual selection arising from fertilization success, with overall shape or presence of “hooks” influencing the ability of a male to successfully inseminate a female.
Justa and Brian explored the shape of these structures by comparing mosquitofish in blue holes with and without predators. Look carefully at the above images – the tip in panel c (a mosquitofish from a high predation site) is slightly longer than the one in panel d (a mosquitofish from a site with no predators). Justa and Brian hypothesize that even this subtle difference could enhance fertilization success for males in the rapid, unsolicited (this is getting really racy), mating events in sites with high predation risk. Males must swoop in quickly and fertilize females before they are eaten by predators; this particularly long gonopodium tip may be just enough to increase fertilization success when more extended copulations are not possible. As such, males with slightly longer tips may have more offspring in high predation environments (even though overall gonopodium length tends to be shorter in high predation areas). Mosquitofish sex isnt nearly as simple as it seems – size AND shape matter.
The technical summary of the paper follows…
Male genital morphology is remarkably diverse across internally fertilizing animals, a phenomenon largely attributed to sexual selection. Ecological differences across environments can alter the context of sexual selection, yet little research has addressed how this may inﬂuence the rapid, divergent evolution of male genitalia. Using the model system of Bahamas mosquito-ﬁsh (Gambusia hubbsi) undergoing ecological speciation across blue holes, we used geometric morphometric methods to test (i) whether male genital shape (the small, approximately 1 mm long, distal tip of the sperm-transfer organ, the gonopodium) has diverged between populations with and without predatory ﬁsh and (ii) whether any observed divergence has a genetic basis. We additionally examined the effects of genetic relatedness and employed model selection to investigate other environmental factors (i.e. interspeciﬁc competition, adult sex ratio and resource availability) that could potentially inﬂuence genital shape via changes in sexual selection. Predation regime comprised the most important factor associated with male genital divergence in this system, although sex ratio and some aspects of resource availability had suggestive effects. We found consistent, heritable differences in male genital morphology between predation regimes: Bahamas mosquito-ﬁsh coexisting with predatory ﬁsh possessed more elongate genital tips with reduced soft tissue compared with counterparts inhabiting blue holes without predatory ﬁsh. We suggest this may reﬂect selection for greater efﬁciency of sperm transfer and fertilization during rapid and often forced copulations in high-predation populations or differences in sexual conﬂict between predation regimes. Our study highlights the potential importance of ecological variation, particularly predation risk, in indirectly generating genital diversity.