Meeting Abstract
We compared patterns of episodic diversifying selection among genes assembled from denovo transcriptomes of Cryptasterina pentagona and C. hystera, which recently speciated (6,000 years ago) and have evolved different mating systems (gonochoric or hermaphroditic), modes of fertilization (outcrossing or selfing), and dispersal (planktonic larvae or internal brooding). We used three complementary models of codon evolution (MEME, BUSTED, aBSREL) to find sites or lineages under positive selection in each gene, and gene ontology methods to characterize the functions of positively-selected genes. The results from this study show that the genes with the largest numbers of sites under selection were linked to immunity (e.g., NLRC5-like) and extracellular-matrix-bound calcium signaling (integrin Fibronectin-like). Other positively-selected genes encoded intracellular calcium signaling of the acrosome reaction (e.g., voltage-dependent calcium channel subunit alpha-2/delta-1) and sperm–egg binding (e.g., egg coat bindin receptor OBi1). Prior work detected little evidence of selection on sperm bindin, and we found that the egg coat bindin receptor EBR1 was partially truncated in C. hystera (but not C. pentagona). These results suggest that the resolution of sexual conflict (via the evolution of selfing hermaphroditism in C. hystera) has led to relaxed selection on some gamete recognition genes, and may have led to reduced specificity of fertilization. The results help to highlight possible targets of selection acting on these species, and warrant carrying out a direct test of gamete compatibility within and between them.
