Schematic drawings of the a) LE ( lessonae-esculentus ) and b) RE ( ridibundus-esculentus ) breeding systems with adults (frog silhouettes), their gametes (eggs and sperm) and the resulting offspring (tadpole silhouettes). Yellow (LL) = P. lessonae, green (LR) = P. esculentus, brown (RR) = P. ridibundus. The Y subscript denotes the male-determining Y factor; all genomes (letters) without subscript carry an X factor. In the LE system, the confinement of the Y factor to the L genome produces female excess among the hybrid offspring. RR offspring typically die before sexual maturity. In the RE system, P. esculentus is all-male in diploid populations.