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, 17 (1), 242

A Reconstruction of Sexual Modes Throughout Animal Evolution


A Reconstruction of Sexual Modes Throughout Animal Evolution

Daniel A Sasson et al. BMC Evol Biol.


Background: Although most extant animals have separate sexes, simultaneous hermaphrodites can be found in lineages throughout the animal kingdom. However, the sexual modes of key ancestral nodes including the last common ancestor (LCA) of all animals remain unclear. Without these data, it is difficult to infer the reproductive-state transitions that occurred early in animal evolution, and thus a broad understanding of the evolution of animal reproduction remains elusive. In this study, we use a composite phylogeny from four previously published studies, two alternative topologies (ctenophores or sponges as sister to the rest of animals), and multiple phylogenetic approaches to conduct the most extensive analysis to date of the evolution of animal sexual modes.

Results: Our analyses clarify the sexual mode of many ancestral animal nodes and allow for sound inferences of modal transitions that have occurred in animal history. Our results also indicate that the transition from separate sexes to hermaphroditism has been more common in animal history than the reverse.

Conclusions: These results provide the most complete view of the evolution of animal sexual modes to date and provide a framework for future inquiries into the correlation of these transitions with genes, behaviors, and physiology. These results also suggest that mutations promoting hermaphroditism have historically been more likely to invade gonochoristic populations than vice versa.

Keywords: Ancestral state reconstruction; Gonochorism; Hermaphroditism; Reproduction; Sexual mode.

Conflict of interest statement

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Competing interests

All authors declare that they have no competing interests.

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Fig. 1
Fig. 1
Ancestral-state reconstruction of sexual modes in animals for both (a) “ctenophore-sister” and (b) “sponge-sister” topologies. For each cladogram, the darkened line indicates the sister lineage to the rest of animals. Numbers in parentheses display the number of animals in each lineage used in the analyses. Circles at nodes depict the states assigned by stochastic character mapping while boxes display the maximum parsimony-assigned states. Blue represents gonochorism, red represents hermaphroditism, and yellow represents asexuality. Boxes evenly divided between red and blue colors indicate that gonochorism and hermaphroditism are equally parsimonious explanations of the data. The grey arrows point to the nodes representing the most recent common ancestor of extant animals. Major clades have been collapsed in this figure. The uncollapsed versions are available as Additional file 4: Figures S3, Additional file 5: Figures S4, Additional file 6: Figures S5, Additional file 7: Figures S6
Fig. 2
Fig. 2
Comparison of selected nodes on stochastic character mapping trees with and without estimated branch lengths. For each node (e.g., Animal LCA) the ctenophore-sister and sponge-sister percentages are represented for the analyses with estimated branch lengths and the ones with no branch lengths (i.e. equal branch lengths). Asexual is not represented here because it was 0.00 at each of these nodes. ctenosis = ctenophore sister; spongesis = sponge sister; LCA = last common ancestor; br.lens. = branch lengths

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