Species boundaries in marine invertebrates
Marine broadcast spawners release gametes into the water column, where fertilization occurs. Because copulatory and other behavioral barriers are absent in these animals, reproductive isolation often occurs as a byproduct of the rapid evolution of sperm and egg proteins. Therefore, studying the evolution of these proteins lends direct insight into the speciation process.
My study organisms are ascidians, particular two species in the genus Ciona: Ciona intestinalis and Ciona robusta (Nydam and Harrison 2007). Ciona robusta is native to the Western Pacific Ocean and has invaded many of the world's temperate oceans. Ciona intestinalis is native to the Northeast Atlantic Ocean and has invaded the Northwest Atlantic Ocean. These two types exhibit partial post-mating prezygotic and postzygotic reproductive isolation.
I examined the previously unknown genealogical relationships among Ciona species using mitochondrial (Nydam and Harrison 2007) and nuclear (Nydam and Harrison 2010a) markers. I then investigated the distribution of Ciona robusta and Ciona intestinalis in the zone of sympatry (the English Channel and the Atlantic coast of France) in 2007 and 2009. I found that these species exhibit low levels of introgression (Nydam and Harrison 2010b). My students and I sampled this hybrid zone again in 2013, and are comparing the introgression results to those obtained from 2007 and 2009 samples. We have recently identified a second hybrid zone on the northern coast of Spain, and are working to characterize the distribution of the two species and the patterns of introgression between them.
Individual boundaries in marine invertebrates
In the benthic oceanic environment, space is often at a premium. Because space is so limited, a process called fusion has evolved in many types of marine invertebrates. Fusion is two separate individuals physically joining to become a single individual. In this environment, where individuals are in close proximity and fusion can occur, how are individual boundaries determined? In the ascidian Botryllus schlosseri, fusion is controlled by two adjacent genes, FuHC secreted and FuHC transmembrane (Nydam et al. 2013a). Individuals that share one or more FuHC secreted alleles will fuse. Individuals can discriminate kin from hundreds of unrelated neighbors, and FuHC is therefore highly polymorphic. I have addressed two questions in Botryllus schlosseri which will increase our understanding of the evolution of allorecognition loci: how is FuHC variation created, and how is it maintained? (Nydam and De Tomaso 2011), (Nydam, Taylor and De Tomaso 2012). I have also studied the evolution of three other groups of genes within the FuHC locus: the fester loci (Nydam and De Tomaso 2012), Hsp40-L (Nydam et al. 2013 (DCI)), and BHF (Taketa et al. 2015, in prep). My students and I are currently examining the evolution of FuHC secreted in Botryllus schlosseri and two other members of the Botryllus schlosseri cryptic species complex: Clade D and Clade E.