Molecular and Developmental Signatures of Genital Size Macro-Evolution in Bugs

Abstract

Our understanding of the genetic architecture of phenotypic traits has experienced drastic growth over the last years. Nevertheless, the majority of studies associating genotypes and phenotypes have been conducted at the ontogenetic level. Thus, we still have an elusive knowledge of how these genetic-developmental architectures evolve themselves and how their evolution is mirrored in the phenotypic change across evolutionary time. We tackle this gap by reconstructing the evolution of male genital size, one of the most complex traits in insects, together with its underlying genetic architecture. Using the order Hemiptera as a model, spanning over 350 million years of evolution, we estimate the correlation between genitalia and three features: development rate, body size, and rates of DNA substitution in 68 genes associated with genital development. We demonstrate that genital size macro-evolution has been largely dependent on body size and weakly influenced by development rate and phylogenetic history. We further revealed significant correlations between mutation rates and genital size for 19 genes. Interestingly, these genes have diverse functions and participate in distinct signaling pathways, suggesting that genital size is a complex trait whose fast evolution has been enabled by molecular changes associated with diverse morphogenetic processes. Our data further demonstrate that the majority of DNA evolution correlated with the genitalia has been shaped by negative selection or neutral evolution. Thus, in terms of sequence evolution, changes in genital size are predominantly facilitated by relaxation of constraints rather than positive selection, possibly due to the high pleiotropic nature of the morphogenetic genes.

Keywords: DNA; development; genitals; morphogenesis; sequence evolution.

Fig. 1.

Phylogenetic diversity of genital size (inner circle), body size (middle circle), and development rate (outer circle) across the Hemiptera, distributed in the ultrametric super-tree constructed here. There is a clear trend of larger species exhibiting larger genitals (e.g., Pentatomoidea and Sternorrhyncha), while development rate seems less correlated with both morphological traits.

Fig. 2.

Relationship between body and genital size (given in mm, log-transformed), where each point is a hemipteran species and point size represents the development rate. Best fit line (orange) is from the GLS analyses of the pure allometric model (see table 1). Fit from the SMA analysis is expressed in purple. One of the species analyzed (Chinavia ubica, with genitalia detached) is highlighted with its genital capsule to illustrate the measured traits (light blue bars).

Fig. 3.

Results of phenotype–genotype association (body-size-corrected genital width versus omega [ω]) estimated in Coevol, showing the degree of correlation versus the posterior probability (y axis). Each point represents a gene, and the dashed line is the posterior probability cut-off of 0.95. Omega values for each gene with significant correlation with the phenotype are represented by point colors and were taken from the site-wise model from CodeML (the mean omega of all codons). (A) Genes with predicted genital development functions; (B control, “non-genital” genes.

Fig. 4.

Selective regime estimated in CodeML for each codon (colored points) of all genes that showed significant association with genital size (detected in Coevol). The vertical dashed line approximates the positive selection zone threshold, where dN/dS are equal (ω = 1).