Catching the phylogenic history through the ontogenic hourglass: a phylogenomic analysis of Drosophila body segmentation genes.

The phylogenetic information content of different developmental stages is a long-standing issue in the study of development and evolution. We performed phylogenetic analyses of 51 body segmentation genes in 12 species of Drosophila in order to investigate the impact of the mode of evolution of development on phylogeny inference. Previous studies of these genes in Drosophila using pairwise phenetic comparisons at the species group level revealed the presence of an "hourglass model" (HG), wherein mid-embryonic stages are the most evolutionarily constrained. We utilized two character-based approaches: taxonomic congruence using the relative consensus fork index (RCFI), in which phylogenies are inferred from each gene separately and compared with a total evidence tree (TET), and partitioned simultaneous analysis using several indices such as branch support (BS) and localized incongruence length difference (LILD) test. We also proposed a new index, the recapitulatory index (R), which divides the number of synapomorphies on the total number of informative characters in a data set. Polynomial adjustment of both BS and R indices showed strong support for the hourglass model regardless of the taxonomic level (species subgroup vs. subgenera), showing less phylogenetic information content for mid-developmental stages (mainly the zygotic segment polarity stage). Significant LILD scores were randomly distributed among developmental stages revealing the absence of differential selective constraints, but were significantly related to chromosomal location showing physical (linkage) impact on phylogenetic incongruence. RCFI was the most sensitive measure to taxonomic level, having a convex parabola at the species subgroup level in support of the hourglass model and a concave parabola at the subgeneric level in support of the adaptive penetrance model. This time-dependent discrepancy of best fit developmental model parallels previous conflicting results from the vertebrates. Because of the quasi-phenetic nature of this index, we argue that the discrepancy is due to the evolutionary rate heterogeneity of developmental genes rather than to fundamental differences among organisms. We suggest that simultaneous character-based analyses give better macroevolutionary support to the hourglass model of the developmental constraints on genome evolution than pairwise phenetic comparisons.