August 24, 2016 report
Genetic study of skinks suggests extreme matrotrophy evolved only once in Africa
(Phys.org)—A team of researchers with Villanova University in the U.S. and associates from South Africa, Germany and Switzerland has found via genetic study that extreme matrotrophy evolved just once in African mabuyine skink. In their paper published in the journal Biology Letters, the researchers describe how they ventured to Zambia and Angola to obtain skink samples and then conducted DNA tests on them to create family trees which allowed them to learn more about the evolutionary history of matrotrophy in skinks.
Skinks are lizards with smooth bodies and short or even absent limbs. Quite often, they look like snakes. In this new effort, the researchers sought to learn more about a certain group of them—those that reproduce using a process called extreme matrotrophy. Instead of simply delivering their young through live birth, or by laying eggs, as is done by most other lizards and snakes, some skinks provide nutrients to their embryos through a placenta—a form of extreme live birth. But has such an ability evolved more than once in different species, or are all such skinks related to a common ancestor? That is what the researchers wanted to know. To find out, they collected multiple samples representing multiple different species, took them back to their lab and set about reconstructing their evolutionary history via genetic study (using multilocus DNA data). They then compared their results with species obtained from other sites in Asia and the Neotropics.
The team reports that testing suggests one likely evolutionary moment that led to matrotrophy in Africa—though they could not rule out the possibility of a second. The data also indicated that there may have been another unassociated moment in the Neotropics. Taken together, the evidence indicates the likelihood that there were no more than three moments (leading to the evolution of three sister groups) that led to matrotrophy in the three groups that make up skink ancestry (which includes 66 species). The results also showed a lot of similarity between the ways mammalian reproduction evolved and placental development in skink ancestors.
Most mammals and approximately 20% of squamates (lizards and snakes) are viviparous, whereas all crocodilians, birds and turtles are oviparous. Viviparity evolved greater than 100 times in squamates, including multiple times in Mabuyinae (Reptilia: Scincidae), making this group ideal for studying the evolution of nutritional patterns associated with viviparity. Previous studies suggest that extreme matrotrophy, the support of virtually all of embryonic development by maternal nutrients, evolved as many as three times in Mabuyinae: in Neotropical Mabuyinae (63 species), Eumecia (2 species; Africa) and Trachylepis ivensii (Africa). However, no explicit phylogenetic hypotheses exist for understanding the evolution of extreme matrotrophy. Using multilocus DNA data, we inferred a species tree for Mabuyinae that implies that T. ivensii (here assigned to the resurrected genus Lubuya) is sister to Eumecia, suggesting that extreme matrotrophy evolved only once in African mabuyine skinks.
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