It has been debated for a while…are males really necessary? Find out how fish of the genus Chrosomus solve the small problems associated with asexual reproduction, in the Early View paper “Diets of sexual and sperm-dependent asexual dace (Chrosomus spp.): relevance to niche differentiation and mate choice hypotheses for coexistence” by Jonathan A Mee and co-workers.
Here’s a short summary of the study:
In order to persist, sperm-dependent asexuals must be ecologically divergent and/or more sperm-limited compared to their sexually reproducing hosts.
It’s easy to be fascinated by sperm-dependent asexual species – they’re one of those oddities of natural history that, collectively, are the reason many of us became students of biology. These all-female “Amazons” require a male (or just their sperm) of a sexually reproducing host species to provide the trigger for egg development and reproduction, but, in most cases, discard the male’s genome.
In addition to being a fascinating natural oddity, the existence of sperm-dependent asexuals raises an interesting scientific question: how do sperm-dependent asexuals coexist with their host species? All-female asexuals have a great advantage over sexuals – by producing no males, the asexuals have twice the potential population growth rate relative to the sexuals. But, in the case of sperm-dependent asexuals, vastly outcompeting the sexuals (i.e., eliminating the source of sperm) would eliminate the ability to reproduce. There are two general mechanisms by which sperm-dependent asexuals and their sexual hosts can achieve stable coexistence. First, sufficient ecological divergence between the sexuals and asexuals would avoid competitive exclusion. Second, if males prefer mating with sexual females rather than asexual females, asexual females would be more sperm limited (and have reduced reproductive output) relative to sexual females.
Our contribution to understanding how sperm-dependent asexuals coexist with their sexual hosts examined the combined influence of these two mechanisms (ecological divergence and male mate choice) on the coexistence of sperm-dependent asexual species and their sexual hosts – previous work only considered each mechanism independently. We integrated the insights of mathematical modeling and empirical data on ecological divergence (from two natural populations of the sperm-dependent asexual fish, Chrosomus eos-neogaeus) to conclude that a combination of both mechanisms may be required for coexistence. This integrated approach is valuable to understanding many ecological and evolutionary processes.