How fish eggs are preadapted for the evolution of matrotrophy.

Area of Science:

• Evolutionary Biology
• Reproductive Strategies
• Comparative Physiology

Background:

• Livebearing (viviparity) has evolved independently multiple times in teleost fishes and squamate reptiles.
• Matrotrophy, a form of livebearing where the mother nourishes the embryo, is common in fish but rare in reptiles.
• The discrepancy in matrotrophy evolution suggests underlying differences in reproductive physiology.

Purpose of the Study:

• To investigate if differences in egg permeability to maternal resources explain the divergent evolution of matrotrophy between fish and reptiles.
• To characterize the mechanisms of nutrient uptake in the eggs of Atherinomorpha fishes.

Main Methods:

• Experiments using oviparous Atherinomorpha fish eggs to assess the uptake of small organic molecules.
• Utilized radiolabeled amino acids to measure uptake rates and test for competitive inhibition.
• Investigated the transport of larger microspheres at varying temperatures to infer the mechanism (pinocytosis) and its temperature dependence.

Main Results:

• Atherinomorpha fish eggs actively transport small organic molecules against a concentration gradient.
• Uptake is non-specific, indicated by similar transport rates for L-leucine and D-leucine.
• Eggs transport larger particles via temperature-dependent pinocytosis, suggesting an active process.

Conclusions:

• The ability of fish eggs to actively acquire maternal resources provides a physiological basis for the evolution of matrotrophy.
• Amniote eggs, lacking direct access to maternal resources, face different evolutionary constraints on developing matrotrophy.
• Egg permeability to maternal resources is a critical factor influencing the evolutionary trajectory of livebearing and matrotrophy.