High heterogeneity in macroevolutionary dynamics of sperm components in perching birds

Explaining the exceptional morphological diversity of spermatozoa remains a challenge because the evolutionary processes acting on sperm components are complex and multivariate. Identifying shifts in evolutionary trajectories across a phylogeny is a powerful approach to address this challenge. Here, we examine macroevolutionary dynamics of avian sperm while also considering coevolution among sperm components (i.e. length of head, midpiece and flagellum). In a dataset of 755 perching bird species (order Passeriformes), we find that evolutionary processes are highly heterogeneous. Passerida (a large clade sensu Sibley and Ahlquist, consisting of about 3,800 species) stands out for its long flagella and midpieces and particularly strong stochastic evolutionary correlations between these components. Estrildid and Viduid finches show strongly bounded evolution of relatively short sperm components (following a strong Ornstein-Uhlenbeck model), in contrast to other Passerida groups where trait variance appears to accumulate linearly through time (following a Brownian Motion model). We suggest that variation in sperm physiology and selective pressures promoting long midpiece and flagellum may be key to a deeper understanding of these macroevolutionary regimes.