Sexual selection in relation to pest-management strategies.

The application of principles derived from the sexual selection literature can assist attempts to subvert the normal mating behavior of pests. Sexual selection encompasses both intermale competition for access to females and female choice of mates. It can operate during long-range attraction and short-range courtship, as well as after copulation. We review the major aspects of sexual selection and illustrate their application to pheromonal and SIT pest-management programs. Pheromones are important both in long-range attraction and in close-range mate choice; parapheromones may be very useful in pest management because of their influence on male mating success. Sexual selection theory provides a scheme for studying the normal mating behavior of a pest species and thus determining which attributes of the mass-reared sterile males are critical to their success with wild females. We hope that our review will suggest novel ways of attacking pests as well as encourage behavioral ecologists to study pest species.

NATURAL HYBRIDIZATION BETWEEN THE SYMPATRIC HAWAIIAN SPECIES DROSOPHILA SILVESTRIS AND DROSOPHILA HETERONEURA.

Two newly formed, morphologically distinct species of Drosophila from the island of Hawaii have been found to form fertile hybrids in two areas of sympatry. Both F1 and backcross hybrids have been recognized in nature; in one case, the hybridization events extended over three years. Original hybridizations involved one or more D. silvestris females mating with D. heteroneura males. Female F1 hybrids from this cross have participated in backcrosses to D. silvestris. In any one locality, less than 2% hybrids have been found in nature. A hybrid swarm was not formed; selection appears to favor a strict maintenence of morphologies characteristic of the separate species. This result is attributed to pervasive sexual selection, which serves to preserve the syndromes of sexual characteristics that arose during past allopatric divergence. Populations of D. silvestris both within and outside the present range of D. heteroneura often display heritable variation in color patterns involving the abdomen, pleurae, legs, and wings. Genes effecting variation in these characters may be derived from genes involved in a past introgression from D. heteroneura. Independent evidence for past hybridization between these species comes from study of mitochondrial DNA. Although the inferred direction of the cross is the opposite of that observed in the recent case described here, both reciprocal crosses have been obtained experimentally in the laboratory. Accordingly, we suggest that these species may have been open to hybridization since their first sympatic encounters following their inception in allopatry. That they remain as strictly recognizable morphological entities is due both to their current partial allopatry and to the action of sexual selection in maintaining two separate major modes of efficient reproduction. There is no reason to invoke specific reinforcing selection that has imposed reproductive isolation.

Sexual selection and speciation: Issues raised by Hawaiian Drosophila.

The Drosophila fauna of Hawaii is extraordinarily diverse, representing about 25% of the world's described species. The most notable characteristics which differentiate the species in Hawaii are morphological and behavioral ones used in courtship and mating. These flies are excellent model species for investigating the evolution of sexually selected traits. Hypotheses regarding the associations between species formation and mating behaviour have been formulated as a result of work on this group, leading to further empirical and theoretical research.

Mitochondrial DNA variability in natural populations of Hawaiian Drosophila. II. Genetic and phylogenetic relationships of natural populations of D. silvestris and D. heteroneura.

Restriction site mapping of mitochondrial DNA with 23 restriction endonucleases was used to examine the genetic and phylogenetic relationships of populations of D. silvestris and D. heteroneura from the island of Hawaii. Two morphological races of D. silvestris are known on the island of Hawaii. One has three bristle rows on the tibia of the foreleg and is found on the east side of the island. The other is found on the west side of the island and has the ancestral bristle row character of two rows on the tibia of the foreleg. All D. heteroneura have the ancestral bristle row character state. We demonstrate that mtDNA restriction site analysis can also differentiate the two D. silvestris races, and that the two bristle row D. silvestris are more closely related to D. heteroneura than they are to their three bristle row conspecifics using both distance and character state analysis. Our study (which uses six base recognition restriction endonucleases) is not sensitive enough to determine the phylogenetic relationships of populations within either of the D. silvestris lineages.

Ethological isolation among three species of the planitibia subgroup of Hawaiian Drosophila.

Drosophila heteroneura and D. silvestris are sympatric species living on the island of Hawaii, while D. planitibia is allopatric on the nearby island of Maui. A pronounced ethological isolation is found between the sympatric species, and none between allopatric ones, except that D. planitibia females discriminate against D. heteroneura males. Male hybrids are sterile in allopatric crosses but fertile in sympatric ones. The ethological isolation and the hybrid sterility are uncorrelated. This is expected if premating isolation between closely related species is an ad hoc product of natural selection, while postmating isolation is an incidental result of genetic divergence. Some exceptions to the rule are discussed.