For their senior theses, Conor Powers ’17 and Alexa Horn ’17 worked with Visiting Assistant Professor of Biology Simon Coppard on projects using transcriptomics to reveal the evolution of venom in sea urchins. They presented their findings to an international audience at the Eighth North American Echinoderm Conference that took place in Worcester, Mass., July 9th to 13th.
Powers’ research focused on the evolution and localized expression of Acidic Phospholipase-PLA2-I (AP-PLA2-I), a gene characterized from the pit viper Bothrops pirajai that is expressed in the globiferous pedicellariae and spines of venomous sea urchins.
This gene codes for an enzyme that catalyzes the cleavage of 2-acyl ester bonds from 3-sn-phospholipids, which ultimately induces cell lysis post envenomation. In situ hybridizations revealed that AP-PLA2-I expression is localized in either the stalk or valve venom glands, or in some cases in both sets of glands, with ducts connecting the two.
Powers’ study showed that the AP-PLA2-I gene has codons under positive selection and that this gene has become weaponized in venomous sea urchin lineages. This has primarily occurred in epifaunal species that are more vulnerable to larger predators such as fish and starfish than in infaunal species that live buried in sand and mud.
Horn’s research focused on the evolution and localized expression of the L-rhamnose-binding lectin CSL3 (RBL) gene in 11 sea urchin species that live in different habitats and are therefore exposed to different predators and parasites.
In other organisms, RBLs are major part of molecular recognition systems but are also cytotoxic and involved with opsonization. Horn’s findings demonstrate that the RBL CSL3 gene has evolved under positive selection and her phylogeny revealed that lineages reflect either the presence or absence of venomous spines or venomous pedicellariae and not systematic relationships based on neutrally evolving genes.
Horn’s mRNA expression work showed that the RBL CSL3 gene was expressed in venom glands and also in muscle and membrane tissue, suggesting that RBL CSL3 has a further role that functions outside the venom glands.