Meeting Abstract
Chelicerates are one of the three main groups of extant arthropods, along with myriapods (millipedes and centipedes) and Pancrustacea (insects and crustaceans). While more diverse in previous geologic eras, extant chelicerates consist of three groups: horseshoe crabs (Xiphosura), sea spiders (Pycnogonida) and arachnids (Arachnida). Extant arachnids are almost exclusively terrestrial, Xiphosura and Pycnogonida are restricted to marine environments. The available draft genomes of xiphosurans and some arachnids have shown the presence of gene duplicates, potentially derived from ancestral genome duplications, but the extent, pattern, and tempo of such gene expansions remain poorly understood. The relationships of chelicerate groups have been elusive even with genomic data at hand. A recurrent pattern in these analyses, commonly attributed to systematic biases, breaks the monophyly of Arachnida by including xiphosurans as a nested clade, albeit with unstable affinities. We used combination of genomic data (transcriptomes and genomes) to identify gene families and produce a species tree based on traceable hypotheses of orthology. The resulting species tree echoes previous results by placing Xiphosura within the arachnid clade. Here, we use the composition and relationship of these individual gene copies to investigate the extent, pattern and tempo of the gene expansion with emphasis on representative gene families. Based on the empirical species tree, we conducted coalescent gene-tree simulations to evaluate the potential effects of incomplete lineage sorting on our ability to distinguish competing hypotheses. Our main goal is to investigate the link of the observed gene expansions and their potential role in macroevolutionary phenomena such as the transition to terrestrial environments.
