The significance of mating processes for the genetics and for the formation of resting eggs in monogonont rotifers

Crossbreeding experiments with three geographically distinct strains (E, S, and L) of the rotifer Brachionus plicatilis have been carried out in an attempt to elucidate the apparent male sterility of strain L, which is unable to produce resting eggs.The 9 crossing possibilities of the three strains have been investigated in 27 experiments. The results refute the concept of male sterility. L-males copulate successfully with mictic as well as with amictic females of strains E and S. Fertilized amictic E and S females produce defective resting eggs, which have only one thin shell and, which disintegrate after deposition. L-females cannot, as a rule, be fertilized. Moreover, crosses between strain E and S are succesful only in one direction; the reciprocal crosses failed.To explain the present results a hypothesis is suggested that the thickness of the body wall of newborn females differs in the three strains, and between mictic and amictic individuals. A sequence of gradually increasing body wall thickness of all types of females involved, together with a comparably increasing penetration ability of the males of strain E, S, and L explains the success or failure of all crosses, including the unilateral cross E × S.