Data from: How fish eggs are preadapted for the evolution of matrotrophy

Teleost fishes evolved livebearing via egg retention 14 times. Matrotrophy
has evolved within 12 of those lineages. By contrast, squamate reptiles
evolved livebearing over 115 times, but only two to four of those lineages
are known to have evolved matrotrophy. Is the discrepancy between these
organisms in the probability of this transition caused by differences in
their eggs? We show that the eggs of oviparous species in the superorder
Atherinomorpha can acquire small organic molecules from their surrounding
environment against a concentration gradient via mechanisms of active
transport. Uptake rates were inhibited by competing radiolabelled amino
acids against unlabelled versions of themselves. Transport was
non-specific as uptake rates were similar for l-leucine and its
biologically uncommon enantiomer d-leucine. Eggs are also capable of
transporting larger microspheres across the membrane, but transport is
inhibited at temperatures below 4°C, suggesting active transport occurs
via pinocytosis. Conflict theory predicts that the ability of the egg to
acquire maternal resources will facilitate the embryo–parent arms race
that leads to the evolution of matrotrophy following the transition to
livebearing. The shelled eggs of amniotes lack such access to maternal
resources when retained in the evolution of viviparity.