Cytogenetic study of three closely related species of Hawaiian Drosophila

Abstract

Three allopatric species from the Hawaiian islands, Drosophila bostrycha (Molokai), D. affinidisjuncta (West Maui), and D. disjuncta (East Maui), are extremely similar in morphology but differ in metaphase chromosomes by the amount and distribution of heterochromatin. Their polytene chromosomes are virtually homosequential with only slight differences at the tip of the microchromosome. Each is polymorphic for one or more inversions, especially in chromsome 4. Salivary gland chromosomes of F1 larvae reared either from wild-caught females or wild-caught males mated to standard laboratory stocks were examined for gene arrangements. Drosophila bostrycha and D. affinidisjuncta share a polymorphism for inversion 4v, which is much more frequent in the latter than in the former. In D. disjuncta 4v has been found only joined in a haplotype with three other inversions (g2 h2 i2) at a low frequency at Kipahulu Valley. Drosophila disjuncta is unique in having another fourth chromosome inversion, 4k, which is highest in frequency at Waikamoi. A new inversion, 2s, was discovered at Uluini Stream. Interspecific hybridizations were carried out in small mass matings. Backcrosses and dissections demonstrated that all F1 females were fertile. All F1 males were sterile in either of two categories with reciprocal hybrids uniformly manifesting one or the other type. Attempts to model the sterility mechanism suggest that more than chromosomal sterility is involved. Our results are discussed in relation to other closely related clusters of species having heterochromatin-based karyotype variations. Key words: heterochromatin, hybrid sterility, inversion, polymorphism, species divergence.