Mating Preferences and Sexual Isolation Within and Between the Incipient Species of Drosophila Paulistorum

Abstract

The male choice experimental method was used to test mating preferences of six races of Drosophila paulistorum. The races (number of strains in parentheses) are: Centro-American (3); Amazonian (4); Orinocan (5); Andean-South Brazilian (6); Transitional (4); and Guianan (1). Strains of the same race often showed significant preferences for homogamic matings, but strains of different races showed such preferences to a much greater extent. On the average, sexual isolation was lower in matings between the Transitional race strains and strains of other races than it was in matings between strains of the other five races. The degree of sexual isolation showed only a weak positive correlation with fertility or sterility of the hybrids between the strains crossed. An addendum contains additions and corrections to the data reported (1959) on D. paulistorum by Dobzhansky and Spassky with regard to the production of sterile and fertile F, hybrids in various crosses between strains. Drosophila paulistorum Dobzhansky and Pavan is a taxon which contains an extraordinary complex of geographic races or incipient species. Five of them have almost reached the status of reproductively isolated but morphologically indistinguishable species. These are the Centro-American, Orinocan, Amazonian, Andean-South Brazilian, and Guianan races. They are mostly allopatric, but in some places their geographic ranges overlap, and two incipient species may coexist sympatrically without interbreeding. When placed together in laboratory experiments, females of one and males of another race exhibit a pronounced and in some instances nearly complete, sexual isolation. If hybrids are produced, the females are fertile but the males are sterile. The sixth, or Transitional, race occurs in Colombia; all strains of this race can be crossed and will produce fertile hybrids of both sexes with at least one of the other five races (Dobzhansky and Spassky, 1959). The two isolating mechanisms, sexual isolation and hybrid sterility, are quite different in their genetic bases. The former is determined by polygenic differences between the races; the latter involves a remarkable predetermination of the egg cytoplasm by the genotype present in the egg before meiosis an,d fertilization (Ehrman 1960, 1961). It is mainly because of the existence of the Transitional race that Drosophila paulistorum is regarded as a single species. It still has a single, though deeply cleft, gene pool. The races (incipient species) of 67 This content downloaded from 157.55.39.123 on Mon, 18 Jul 2016 05:57:34 UTC All use subject to http://about.jstor.org/terms 68 THE AMERICAN MIDLAND NATURALIST 68(1) D. paulistorum are among the best known examples of borderline situations bridging the gap between race and species (Dobzhansky, 1961). Analysis of such borderline situations presents a challenging problem to evolutionists. It is highly improbable that the reproductive isolation which is observed between the incipient species could have arisen simultaneously as an absolute barrier to gene exchange. More likely, it arose gradually, and is probably in the process of construction now. \;: :,IONNDURAs SALVADO OSTA RICA ~ NAM 5NT MARTA 9 T~~ A6 BUCARAMANGA. BUENAVENTURA* *TRINIDAD P0NR LLANSANT )MINGGO