Proposed evolutionary model of the metazoan Class 2 B1 receptor genes.

<p>The five major metazoan Class 2 B1 receptor gene phylogenetic clusters are represented by filled colored dots according to their proposed common origin in the bilateral genome (please refer to symbols list). For simplicity, the species-specific gene duplications/deletions of the members within each receptor family are not represented. We propose that the cluster A ancestral gene emerged early in the nematode and arthropod radiation while cluster B is proposed to already be present in the bilateral genome. The ancestral PDF-R/PDF-R-related cluster gene duplication occurred prior to the nematode-arthropod divergence and in the arthropod lineage selective gene deletions occurred. The PDF-R gene has probably been deleted from arachnidan while PDF-R-related gene has been eliminated from the other arthropod genomes. The ancestral DH31-R/Hec-R gene has been deleted in the nematode lineage and Hec-R gene emerged in the insect lineage. Major divergence time points proposed during metazoan evolution in millions of years ago (Mya) are indicated and are taken from; a <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0092220#pone.0092220-RotaStabelli1" target="_blank">[64]</a>, b <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0092220#pone.0092220-You1" target="_blank">[65]</a> and c <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0092220#pone.0092220-Hedges1" target="_blank">[66]</a>. The two rounds of genome duplication (1R and 2R) in the deuterostome radiation are represented. The PDF-R-related gene has probably been deleted from the chordate lineage prior to 1R. The figure is not designed to scale.