Sperm mixing in the Alpine newt (Triturus alpestris)

In polyandrous females the ultimate stage of cryptic female choice may involve egg–sperm interactions during different phases of fertilization. This form of sperm discrimination is possible only when sperm from different males have simultaneous access to eggs at the site of fertilization. In polyandrous newts of the genus Triturus, eggs are fertilized internally by sperm stored for an extensive period of time in the tubular spermatheca. The extent of sperm mixing, which is a necessary condition for cryptic female choice involving sperm–egg interactions, was studied in doubly mated female Alpine newts, Triturus alpestris. Using an allozyme marker the paternity of offspring sired by the two males was established in both series of larvae reared from eggs produced consecutively over short period of time (ca. 2 h) and batches of eggs collected during longer periods of time (up to 26 days). Significant sperm mixing was unequivocally demonstrated by the mixed paternity of the progeny produced in series. The paternity pattern in batches of eggs collected during longer periods of time showed neither significant predominance of either male in the progeny nor any effects of sperm stratification in the tubules of the spermatheca.