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Parallel evolution of placentation in Australian scincid lizards
James R Stewart1, Michael B Thompson
1Department of Biological Sciences, East Tennessee State University, Johnson City, Tennessee 37614, USA. stewarjr@etsu.edu
Summary
Viviparity evolved independently in related lizards, Pseudemoia and Niveoscincus. Their placentas show similar structures due to parallel evolution and developmental constraints, enabling nutrient and respiratory exchange.
Area of Science:
- Evolutionary Biology
- Reproductive Biology
- Herpetology
Background:
- Viviparity and placental nutrient provision have evolved multiple times in squamate reptiles.
- Five lineages are substantially placentotrophic, including two closely related scincid lizard genera, Pseudemoia and Niveoscincus.
Purpose of the Study:
- To investigate the independent evolution of viviparity and placentation in Pseudemoia and Niveoscincus.
- To compare placental ontogeny and identify homoplasy in these closely related genera.
Main Methods:
- Histological comparison of placental development in placentotrophic species.
- Analysis of extraembryonic membrane development in oviparous outgroups.
Main Results:
- High levels of homoplasy observed in placental evolution.
- Terminal placentas consist of omphaloplacentas and chorioallantoic placentas with both shared and unique characters.
- Three derived placental functions inferred: respiratory exchange (chorioallantoic), nutritive exchange (omphaloplacental), and localized nutritive exchange (chorioallantoic).
Conclusions:
- Structural similarities in chorioallantoic placentae for respiration likely arose from parallel evolution driven by selection to maintain ancestral egg functions.
- Omphaloplacental similarities are explained by selection for nutrient transport constrained by yolk mass development.
- Chorioallantoic nutritive sites evolved independently and differ structurally due to a lack of developmental constraints.
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