Abstract
Most of the present knowledge on animal reproductive mode evolution, and possible factors driving transitions between oviparity and viviparity is based on studies on vertebrates. The species rich door snail (Clausiliidae) subfamily Phaedusinae represents a suitable and unique model for further examining parity evolution, as three different strategies, oviparity, viviparity, and the intermediate mode of embryo-retention, occur in this group. The present study reconstructs the evolution of reproductive strategies in Phaedusinae based on time-calibrated molecular phylogenetics, reproductive mode examinations and ancestral state reconstruction. Our phylogenetic analysis employing multiple mitochondrial and nuclear markers identified a well-supported clade (including the tribes Phaedusini and Serrulinini) that contains species exhibiting various reproductive strategies. This clade evolved from an oviparous most recent common ancestor according to our reconstruction. All non-oviparous taxa are confined to a highly supported subclade, coinciding with the tribe Phaedusini. Both oviparity and viviparity occur frequently in different lineages of this subclade that are not closely related. During Phaedusini diversification, multiple transitions in reproductive strategy must have taken place, which could have been promoted by a high fitness of embryo-retaining species. The evolutionary success of this group might result from the maintenance of various strategies.
Highlights
The evolution of novel phenotypes that provide access to new ecological niches can increase the diversification rate of lineages (Yoder et al, 2010)
Most of the present knowledge on animal reproductive mode evolution, and possible factors driving transitions between oviparity and viviparity is based on studies on vertebrates
The present study reconstructs the evolution of reproductive strategies in Phaedusinae based on time-calibrated molecular phylogenetics, reproductive mode examinations and ancestral state reconstruction
Summary
The evolution of novel phenotypes that provide access to new ecological niches can increase the diversification rate of lineages (Yoder et al, 2010). Viviparity might have evolved as an adaptation to cold climate where viviparous females can use thermo regulatory behaviour to shorten embryonic developmental time and to reduce the exposure of embryos to stressful environmental conditions (Shine, 1983) This so-called cold-climate hypothesis, linking viviparity with lower temperatures characteristic for high latitudes and altitudes, acquired much but not unequivocal support (Hodges, 2004, Watson et al, 2014, Ma et al, 2018). This explanation is likely not applicable to most non-squamate animals, yet the herpetological perspective pre dominates in studies on the evolution of parity. The relatively common occurrence of this reproductive mode in freshwater snails as opposed to marine snails (Glaubrecht, 2006), can be seen as an adap tation to a harsher and less predictable habitat (Kohler et al, 2004)
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