This week’s meeting focused on asexuality. Last week we defined asexuality, for the purposes of this seminar, as reproduction by an all-female (single sex) species that does not require fertilization by sperm. This week, we delved into the life cycles of several organisms that rely on asexuality for some or all of their reproduction.Why has asexuality not completely taken over?
To start, we revisited on an idea from last week: the cost of producing males. Jay mentioned that this idea was specific to populations in which males do not actively promote the fitness/survival of offspring (i.e. in which males do not contribute to parental care). In this context, males provide genetic material, but nothing more. Consequently, sexual reproduction consumes time and energy (say, in finding a mate) without a lot of return benefit to females or offspring, and so keeping males in the population is “costly” in comparison to asexual reproduction. We wondered if males getting eaten post-mating (for organisms for which that occurs) counted as a sufficient contribution for negating that “cost”; our jury is still out!
Why does sexual reproduction still exist despite the cost of producing males? One argument is that sexual reproduction is beneficial because it allows for gene recombination. In other words, sexual reproduction begets genetic diversity, which can increase the possibility of adapting to changing environments. The authors of our first paper (Rispe et al. 1998) argued that gene recombination is a long-term benefit (across multiple generations), but that a short-term benefit is also necessary to explain why asexuality is not universally favored. They suggested that even across single generations, sexual reproduction can enhance offspring survival. To illustrate, they focused on aphids (unfamiliar with aphids? check out the figure and link below). Aphids can reproduce via obligate parthenogenesis (i.e. strictly cloning) or cyclical parthenogenesis (alternating cloned generations with sexual generations). For aphids, sexual reproduction is the only way to produce eggs, which happen to be relatively cold-resistant. So, for latitudes or elevations that have frequent cold winters, cyclical parthenogenesis is common, whereas obligate parthenogenesis reigns in locations that are consistently warmer.
Image 1) Aphids alternate between sexual and asexual reproduction strategies. Image 2) Aphids use their straw-like mouthparts to pierce plants and suck sugary plant fluids. To learn more about aphids and the source of these images, check out this link.
We had a few discussion points about this intriguing system. Because aphids can travel long distances, we talked about the potential effects of migration on the gene pool – it could maintain cyclical or obligate parthenogenesis in populations that might otherwise convert wholly to one or the other. We wondered about the mechanisms behind cyclical parthenogenesis – sexual and asexual reproduction would seemingly entail very different physical development trajectories, so how might a species keep both? (We’re not sure!). We also considered whether the authors’ argument was valid. Is it that ecologically-relevant traits (e.g. cold resistant eggs) are simply connected to and thereby maintain sexual reproduction, or is there actually a true advantage of this mode of reproduction (related to short-term gene recombination)? It’s not possible to separate these confounding variables in this situation, so we’ll have to keep this question open for the future.
How might asexuality come to be?
We next discussed a paper by Vrijenhoek (1989) focused on asexual vertebrates. Most of the author’s examples were reptiles and fishes wherein asexuality arose from hybridizing two sexual species. Hybridization, in those cases, was enough to break the typical cycle of meiosis (preventing gene recombination), but not damaging enough to prevent organism development thereafter. This paper covered a lot of ground, mostly circling around why so-called “unisexual vertebrates” are rare. The author described myriad constraints on the initial creation of hybrid asexuals: even familiar species-crosses can vary in success depending on individual parents, and ecological impediments exist as well. One example of an ecological constraint is pseudogamy, or false fertilization, in which sperm must be present for an egg to continue developing, even though the sperm and egg never fuse. Vrijenhoek noted that gynogenetic organisms (asexual females dependent on mating in this way) are essentially “parasitic” on nearby populations of sexual species.
Around the table, we talked about how and why “unisexual hybrids” might arise, establish, and persist. Part of our conversation focused on discerning any generalizable patterns for asexuality being an advantageous adaptation. For example, could certain types of environments be more likely to drive the development of hybrid asexuality? We weren’t certain, but we thought that driving pressures might include low likelihood of finding conspecific mates, or very stable homogeneous environments (which might reduce advantages of genetic diversity).
We also spent a few minutes organizing vocabulary. Asexual vs. unisexual? In this course, we define unisexual to be a form of sexual reproduction in which gametes are combined but do not vary in size (and so the parents, rather than belonging to separate sexes, might be better described as mating types). Vrijenhoek’s use of unisexual, however, seemed to be synonymous with the term “asexual” as we defined it (see above). We deduced this when Vrijenhoek wrote of unisexual and bisexual species to mean single-sex species versus species with two sexes. A potentially confusing multitude of terms regarding reproduction exists in the literature; we mused that these terms might overlap or differ across phyla, both because the researchers who coined the terms didn’t interact with one other, and because reproductive strategies are (as we are learning) incredibly diverse.
And just when you think your terms are organized neatly enough, fungi make you throw them out the window.
Our last paper, Anderson and Kohn (1998), was a quick glimpse into the classical-theory-defying realm of reproduction in fungi. It introduced the idea that genetic recombination can, in fact, take place without sexual forms of reproduction. In fungi, cells (that are not gametes) from different individuals can fuse with one another, allowing different cell nuclei to occupy the same space. At this point, the nuclei can themselves fuse, and genetic recombination can take place. Anderson and Kohn suggested that the complexity of reproduction in fungi might make them ideal research subjects for population genetics. As Jacob pointed out, this puts an unusual spin on the idea of model organisms (typically prized for their simplicity). Perhaps that’s just as well for organisms that counter traditional ideas in so many ways.
Having landed temporarily here, among organisms in which recombination can occur without the mating of morphologically distinct sexes, we now look forward to next week. Topic: sexual reproduction in species with one sex (isogamy) and multiple sexes (anisogamy)!
Papers for this week:
- Rispe, C., Pierre, J. S., Simon, J. C., & Gouyon, P. H. (1998). Models of sexual and asexual coexistence in aphids based on constraints. Journal of Evolutionary Biology, 11(6), 685-701. PDF
- Vrijenhoek, R. C. (1989). Genetic and ecological constraints on the origins and establishment of unisexual vertebrates. Evolution and ecology of unisexual vertebrates, 466, 24-31. PDF
- Anderson, J. B., & Kohn, L. M. (1998). Genotyping, gene genealogies and genomics bring fungal population genetics above ground. Trends in ecology & evolution, 13(11), 444-449. PDF