The origin of the Hox/ParaHox genes, the Ghost Locus hypothesis and the complexity of the first animal

Abstract

A key aim in evolutionary biology is to deduce ancestral states to better understand the evolutionary origins of clades of interest and the diversification process(es) that has/have elaborated them. These ancestral deductions can hit difficulties when undetected loss events are misinterpreted as ancestral absences. With the ever-increasing amounts of animal genomic sequence data, we are gaining a much clearer view of the preponderance of differential gene losses across animal lineages. This has become particularly clear with recent progress in our understanding of the origins of the Hox/ParaHox developmental control genes relative to the earliest branching lineages of the animal kingdom: the sponges (Porifera), comb jellies (Ctenophora) and placozoans (Placozoa). These reassessments of the diversity and complexity of developmental control genes in the earliest animal ancestors need to go hand-in-hand with complementary advances in comparative morphology, phylogenetics and palaeontology to clarify our understanding of the complexity of the last common ancestor of all animals. The field is currently undergoing a shift from the traditional consensus of a sponge-like animal ancestor from which morphological and molecular elaboration subsequently evolved, to a scenario of a more complex animal ancestor, with subsequent losses and simplifications in various lineages.