Reproductive isolation between two closely related species of the Queensland fruit fly Dracus tryoni (Frogg) and D. neohumeralis Hardy (Diptera : Tephritidae) II. Genetic variation in humeral callus pattern in each species as compared with laboratory-bred hybrids

Abstract

The variation in humeral callus pattern in Dacus tryoni is genetic. The more brown there is in the parents, the more there is in the offspring. The possible range of variation in D. tryoni obtained in a selection experiment is from an almost entirely yellow humeral callus to a completely brown one. The variation in Dacus neohumeralis is also genetic. The range of variation seen in D. neohumeralis in a selection experiment is from an entirely brown humeral callus to a very large yellow area. The possible range of variation extends probably to much more yellow. The actual colour forms obtained were different in the two species. For a given length of the yellow area in D. neohumeralis flies this yellow area is narrower, further away from the posterior margin of the humeral callus, further away from the posterodorsal margin and closer to the frontal margin, than it is in D. tryoni. In D. neohumeralis the yellow is dirty greenish yellow while in D. tryoni it is bright yellow. The range of variation in F1 hybrids between the two species is different again. The darker F1 flies closely resemble D. neohumeralis, in fact most F1 flies from these experiments were of this type. The more yellow F1 flies are very like D. tryoni and the ones intermediate between these two are unlike any D. tryoni or D. neohumeralis seen. The range of variation in F2 hybrids is much greater than in either species. In fact it covers the whole range of both species. Over the entire range of variation one finds some flies that look like variants of D. tryoni, some that look like variants of D. neohumeralis, and again some others that look different. At the darker end of the scale, variants resembling D. neohumeralis predominate while it is variants resembling D. tryoni that are most the frequent on the other end of the scale. The flies obtained from the wild could all easily be classified as either D. tryoni or D. neohumeralis variants. It is concluded that the occurrence of intermediates is no evidence for hybridization between the two species taking place in the field. The evidence available from the literature in favour of the hypothesis that the two species do hybridize is discussed and it is shown that this evidence can also easily be explained under the hypothesis that they are completely isolated reproductively. Selection in D. tryoni in the wild apparently favours flies with some dorsal brown and acts against both extremes, an entirely yellow humeral callus on one hand and more brown on the humeral callus on the other. Selection in D. neohumeralis in the wild apparently favours flies with a mottled brown humeral callus and acts against both plain brown and yellow areas. The nature of the selective factors bringing this about is still unknown.