Cytogenetics of the Parthenogenetic Grasshopper Warramaba (Formerly Moraba) Virgo and its Bisexual Relatives. V. Interaction of W. Virgo and a Bisexual Species in Geographic Contact
The all-female grasshopper species Warramaba virgo (Orthoptera, Eumastacidae, Morabinae) and one of the two bisexual species from which it arose by hybridization (W. sp. 'P196') exhibit a parapatric type of distribution in the vicinity of Kalgoorlie, Western Australia. Colonies of the two species have been found as close as 900 meters apart, but no mixed colonies have been encountered, in spite of the fact that the two species appear to have identical ecological requirements. Mutual exclusion of one species by the other is presumably due to formation of essentially sterile triploid hybrids whenever they have come into contact in the past. Where they are parapatric, the two species are approximately equal in fecundity (number of eggs laid by a female in her lifetime). Species P196 exhibits a marked north-south cline in fecundity, the most northerly individuals laying more than twice as many eggs as the members of the most southerly population. The fecundity cline is paralleled by a cline in overall size, in both sexes. The adaptive significance of the fecundity cline is obscure, but it is probable that W. virgo has been able to expand into the former range of P196 in an area where the fecundity of the latter was minimal. It is concluded from the cytological data (White et al., 1977; Webb et al., 1978) and the distributional data in the present paper that W. virgo arose by hybridization in the first place at a point where the periphery of the range of species P169 intersected that of the range of P196.