The macroevolution of size and complexity in insect male genitalia.

Andrey Rudoy,Ignacio Ribera

doi:10.7717/peerj.1882

Andrey Rudoy, Ignacio Ribera

Open Access

https://doi.org/10.7717/peerj.1882

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Journal: PeerJ	Publication Date: Apr 14, 2016
Citations: 10	License type: CC BY 4.0

Affiliation: Institut de Biologia Evolutiva, Pompeu Fabra University

Abstract

The evolution of insect male genitalia has received much attention, but there is still a lack of data on the macroevolutionary origin of its extraordinary variation. We used a calibrated molecular phylogeny of 71 of the 150 known species of the beetle genus Limnebius to study the evolution of the size and complexity of the male genitalia in its two subgenera, Bilimneus, with small species with simple genitalia, and Limnebius s.str., with a much larger variation in size and complexity. We reconstructed ancestral values of complexity (perimeter and fractal dimension of the aedeagus) and genital and body size with Bayesian methods. Complexity evolved more in agreement with a Brownian model, although with evidence of weak directional selection to a decrease or increase in complexity in the two subgenera respectively, as measured with an excess of branches with negative or positive change. On the contrary, aedeagus size, the variable with the highest rates of evolution, had a lower phylogenetic signal, without significant differences between the two subgenera in the average change of the individual branches of the tree. Aedeagus size also had a lower correlation with time and no evidence of directional selection. Rather than to directional selection, it thus seems that the higher diversity of the male genitalia in Limnebius s.str. is mostly due to the larger variance of the phenotypic change in the individual branches of the tree for all measured variables.

Highlights

Insect genitalia have been the focus of much attention since the middle of the 19th century, when their taxonomic value to diagnose species with otherwise very similar external morphologies was recognised
We study the macroevolution of the size and complexity of male genitalia in a diverse insect lineage with a very uniform external morphology but extraordinary male genital variability, the aquatic beetle genus Limnebius
We studied the evolution of the morphological characters trough the full evolutionary path of species and in the individual branches, using phylogenetic ancestor-descendant comparisons (PAD; Baker et al, 2015)

Summary

Introduction

Insect genitalia have been the focus of much attention since the middle of the 19th century, when their taxonomic value to diagnose species with otherwise very similar external morphologies was recognised. Sexual selection is widely acknowledged as being the major force driving the evolution of animal genitalia (Hosken & Stockley, 2004; Simmons, 2014), but there is still little consensus on what are the dominant factors (such as male competition, male–female sexual conflict or pure female choice) or the main macroevolutionary trends generating their diversity (Simmons, 2014). One of the few widely accepted general trends is the negative allometric scaling of most genital traits (Eberhard et al, 1998; Hosken, Minder & Ward, 2005; Eberhard, 2009), which suggests that genital morphology is often subject to stabilising selection.

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Conclusion