PLASTICITY OF TAXONOMIC CHARACTERS OF THE QUEENSLAND FRUIT FLIES DACUS TRYONI AND DACUS NEOHUMERALIS (TEPHRITIDAE).

Abstract

The fruit flies Dacus tryoni and D. neohumeralis which have a distribution on the eastern side of Australia, are distinguished taxonomically by the color of the humeral callus (Birch, 1961). D. tryoni has a yellow callus and D. neohumeralis has a brown callus. The situation, however, is more complex than this. In natural populations the callus of flies ranges from all brown to all yellow with a complete range of flies with different proportions of yellow and brown in between. Because of this continuum, any division of flies on the basis of color of the callus is quite arbitrary. However, two groups of flies can be distinguished by their mating behavior. One group, which we call D. tryoni, comprising flies with a callus that has from 0% to 3% brown mates at dusk. A second group, which we call D. neohumeralis, having a callus of 100% brown, mates at noon. The mating time (or times) of flies which have an intermediate category of callus color have yet to be determined. Unfortunately this is difficult, for two reasons; firstly because of their low frequency, which is usually less than 10% of the population in the field, and secondly because these fruit flies only mate in a crowd, so a large number is necessary for reliable data on mating time. These facts have been documented by Birch (1961), Lewontin and Birch (1966), Wolda (1967a) and Gibbs (1967). All these authors, except Wolda, have interpreted the existence of intermediate forms as evidence in favor of introgressive hybridization between D. tryoni and D. neohumeralis. The existence of flies outside the distribution of D. neohumeralis that have some brown on a mostly yellow callus has also been interpreted as introgression of genes of D. neohumeralis into populations of D. tryoni. Our interest in the hypothesis of introgressive hybridization is that hybridization could have aided in the rapid selective changes that must have occurred in D. tryoni if this species has spread south from the tropics to colonize temperate regions of Australia, as has been suggested elsewhere (Birch 1965; Lewontin and Birch, 1966). Wolda (1967b) quite correctly pointed out that alternative hypotheses that might account for the existence of intermediate forms of flies had not been falsified by any data presented by Lewontin and Birch (1966). He showed that flies intermediate in callus characters could be obtained from both D. neohumeralis and D. tryoni by selection. His examination of intermediate forms created by selection and intermediate forms collected in the field led him to try to separate field flies into two groups on the basis of patterning of brown and yellow on the humeral callus. However, he concluded that unless we find some characters other than the color of the humeral callus to enable us to distinguish between the two species and between these and hybrids, we are unable in many cases to distinguish between hybrids and variants of either species. We have, therefore, studied four additional quantitative characters. In this paper we report the continuous nature of the variation for each of these characters as well 1 This paper is dedicated to Professor Th. Dobzhansky of The Rockefeller University in recognition of his long and distinguished contribution to studies of evolution in every continent of the world and his inspiration to biologists of all continents.