Sperm cells are the most morphologically diverse animal cells, but how does that diversity arise, and what are its evolutionary consequences? SpESL aimed to understand how sperm morphological diversity evolves and generates biodiversity, integrating across multiple approaches. SpESL investigatee divergence in sperm morphology between partially reproductively isolated intertidal snail ecotypes. Ecotypes are populations that show adaptations to a particular set of environmental conditions. The snails are an excellent model system for understanding how traits diverge, because they have diverged into different ecotypes repeatedly--here I focus on one ecotype that has adapted to avoid predation by crabs on rocky beaches, and another ecotype that has adapted to avoid being washed off steep, exposed cliffs by strong wave action. This species is also a model system for understanding the evolutionary consequences of trait diversification because they continue to interbreed where the two environments meet. It was recently discovered that the shape of the sperm cells also differs between the ecotypes, even though sperm shape has nothing to do with adaptation to crab predation or wave action. Why sperm shape differs is thus a mystery. Intriguingly, this species is highly promiscuous--at one time, a female can have offspring with over 20 sires in her brood pouch! And females can store sperm for a prolonged period. The combination of prolonged storage of sperm and high promiscuity implies that the female reproductive tract could act as a selective filter, favoring the storage and use of sperm with certain characteristics, for example, certain shapes. If females have such a selective filter, they may have a preference for own- ecotype over other-ecotype sperm, but this is unknown. These unknowns were the topics I set out to address in my main project. In addition, I mentored junior researchers investigating how climate change impacts reproduction in this species. Specifically, we developed projects to examine how temperature impacts sperm quantity and shape in this species. Like many other snails, this species actually has two types of sperm cells--long, skinny "eusperm" that have DNA and actually fertilize the egg, and also spherical "parasperm" that appear unable to move on their own. Parasperm are likely just big packets of nutrition that the male delivers to the female during mating, and we hypothesized that, if males are physiologically stressed by high temperatures, they might not be able to invest as much in parasperm as normal. Temperature is broadly known to affect sperm production, but this field has not been studied much in intertidal invertebrates.
Question 1: Why do sperm differ between ecotypes?
Approach/objective: I sequenced the RNA in the testes to find out which genes are expressed. By relating gene expression levels to sperm size, I can determine which genes might be responsible for causing differences in sperm size. We can also trace these genes back to their locations in the genome, to see if the genes that are connected to sperm length are physically close to the genes that are important for adapting to the environment. Such spatial proximity can be important for causing sets of traits to diverge in parallel. This objective also gave me valuable opportunity to increase my skills in data handling, bioinformatics, and computation.
Question 2: Does the divergence in sperm between ecotypes cause reduced interbreeding?
Approach/objective: Here, the plan was to perform controlled laboratory crosses, where each female would mate with only one male from her own ecotype and one male from the other ecotype. By comparing how many offspring each male sired, and relating that information to the size of his sperm, we could evaluate whether there was an own-ecotype sperm advantage in fertilization. This project gave me opportunity to extend my skills in experimental design.
Question 3: Does temperature impact sperm and reproduction?
Approach/objective: I co-mentored junior researchers for these projects, with the other mentor bringing expertise in the female side of reproduction. Junior researchers exposed snails to elevated, but realistic, temperatures over a period of time and evaluated both sperm production and morphology, and development of embryos in the female brood pouch. This experience was valuable for developing my mentorship skills.
