The Johnson lab has another strong showing here at SICB 2017 with lots of presentations and posters! I stopped by two of their (many) posters on the evolution of reproductive behaviors and sexually-selected traits in anoles.
Adam Zeb and Amy Payne presented their poster that compared reproductive behaviors in anoles to the size of the neuromuscular junctions (NMJs) in the muscles responsible for dewlap extension (ceratohyoid) and hemipenes retraction (retractor penis). They predicted that species with higher dewlap extension rates and copulation rates would have larger NMJs because the NMJ is where the neuron communicates with the muscle fiber, initiating contraction. To ask this question they observed and collected 15 species of anoles from the Dominican Republic, Puerto Rico and the Bahamas. For each species they measured dewlap display rate, copulation rate, and also collected the ceratohyoid muscle and the retractor penis muscle. These tissues were sectioned and stained with acetylchloine iodide and lithium iodide to find the and measure the NMJs. This is still a work in progress, but preliminary evidence doesn’t suggest that NMJ area is correlated with retractor penis muscle size or with ceratohyoid size. However, there was a strong difference in NMJ size between those two muscle types, where the ceratohyoid has over 3x larger NMJs than the retractor penis muscle. This somewhat supports their original hypothesis that NMJ size would be correlated with use, as the dewlap is used much more frequently than the retractor penis muscle. Hopefully next SICB we’ll hear more!
Another Johnson lab member, Hannah Hall, has been working on a project to look at the relationship between pre- and postcopulatory traits in Anolis and to characterize the architecture of the Anolis testis. We know that Anolis have highly variable sperm morphology, but we do not know if a portion of that variation may be due to variation in the structure of the testis. To test this, Hannah collected testis sections from eight species of anoles, and measured the cross-sectional area of the seminiferous tubule, the lumen and the epithelial height. She also collected measurements of sexual size dimorphism (as a proxy for the strength of precopulatory selection) and gonadosomatic index (GSI), which is the ratio of testis mass and body mass (as a proxy of the strength of postcopulatory selection). She found a negative correlation between SSD and GSI, suggesting a trade-off between pre- and postcopulatory selection. She also found significant positive correlations between cross-sectional area of the testis and sperm head size, and between lumen size and sperm tail size. This suggests that larger structures in the testis are needed to produce sperm with larger morphology. Hannah is still working on characterizing the testis structure of many anole species, so stay tuned for more developments!