The Liroria ewingi complex (Anura : Hylida) in south-eastern Australia II. Genetic incompatibility and delimitation of a narrow hybrid zone between L. ewingi and L. paraewingi

Abstract

Most hybrid embryos resulting from artificial crosses between females of Litoria ewingi and males of L. paraewingi are characterized by specific developmental abnormalities-acephaly and anophthalmia. This criterion was used to map the area of interaction between these morphologically similar species in south central Victoria by crossing males from different localities to females of L. ewingi. Three categories of males were recognized on the basis of the proportion of anophthalmic embryos in their offspring: L. ewingi, L. paraewingi, and putative hybrids or backcross progeny. Frequencies of developmental abnormalities in field-collected egg masses were also determined and related to the laboratory crosses. The results of these analyses revealed that the species form a narrow zone of interaction, with males of both species being present throughout the zone, as well as presumed hybrids and backcross progeny. The zone of overlap has been plotted from near Narbethong in the south-east to north of Seymour in the north-west (c. 100 km). Its width varies from less than 5 km in the south-east, where it appears to be correlated with a forest-grassland ecotone, to about 11 km near its northwestern limit, where a sharp vegetational gradient is absent. The presumed antiquity of the contact, and the narrowness of the zone, suggest a stable situation. It is postulated that any selection towards reinforcement of premating isolating mechanisms within the zone, and consequent reduction of hybridization, is counteracted by a continual input of naive individuals from adjacent allopatry.