Topping off: a mechanism of first-male sperm precedence in a vertebrate.

Competition among the sperm of rival males is an important evolutionary phenomenon in many organisms. Yet, despite extensive research on sperm competition in some vertebrate taxa, very little progress has been made on this topic in amphibians. Urodele amphibians (newts and salamanders) are of particular interest to theories of sperm competition because most urodele females--in contrast to other vertebrate females--control the transfer of sperm from the male. Here we present a molecular study of sperm precedence and storage patterns in the rough-skinned newt (Taricha granulosa). First, we used microsatellite markers to show that female newts typically use sperm from 1-3 males under natural and seminatural conditions. Second, we mated experimental females sequentially to two males and collected fertilized eggs in a temporal series. Patterns of paternity were consistent with first-male sperm precedence and complete mixing of sperm within the female. This simple pattern of sperm usage, best described as "topping off," is consistent with the expectation from sexual conflict theory that free female choice before insemination eliminates selective pressures for the evolution of complex patterns of paternity manipulation involving cryptic female choice.