Reproductive characteristics of the flower-breeding Drosophila hibisci Bock (Drosophilidae) in eastern Australia: within-population genetic determinants of ovariole number

Population structure and host-plant specialization in two Scaptodrosophila flower-breeding species

In contrast to phytophagous insect species, little attention has been paid to the possibility of host races in the Drosophilidae, although flower-breeding species, where courtship and mating take place on the flowers, are likely candidates. Two species of Scaptodrosophila, S. hibisci and S. aclinata, are restricted to flowers of Hibiscus species (section Furcaria), and the Furcaria specialization likely predated the separation of S. hibisci and S. aclinata. In all, 20 microsatellite loci were analysed in nine populations of S. hibisci and five of S. aclinata. For two pairs of S. hibisci populations in close proximity, but breeding on different Hibiscus species, differentiation between the populations of each of these pairs was similar to that between the populations that were from the same Hibiscus species, but geographically distant, suggesting the early stages of host-race formation. Genetic variability was significantly less in S. aclinata than in S. hibisci, suggesting greater drift effects in the former. However, of 253 alleles detected, 82 were present in both species, 160 in S. hibisci only and 11 in S. aclinata only, indicating that S. aclinata was derived from S. hibisci, following a strong bottleneck at the time of separation--possibly 40,000 years BP. Analyses and interpretation of Hardy-Weinberg equilibrium and F statistics needed to account for null alleles known to be present at eight loci in S. hibisci, and possibly present at other loci. The results emphasize the need for caution in studies where the presence of null alleles is inferred only from population data.

Evolutionary experimentation through hybridization under laboratory condition in Drosophila: evidence for recombinational speciation

BACKGROUND

Drosophila nasuta nasuta (2n = 8) and Drosophila nasuta albomicans (2n = 6) are a pair of sibling allopatric chromosomal cross-fertile races of the nasuta subgroup of immigrans species group of Drosophila. Interracial hybridization between these two races has given rise to new karyotypic strains called Cytorace 1 and Cytorace 2 (first phase). Further hybridization between Thailand strain of D. n. albomicans and D. n. nasuta of Coorg strain has resulted in the evolution of two more Cytoraces, namely Cytorace 3 and Cytorace 4 (second phase). The third phase Cytoraces (Cytorace 5 to Cytorace 16) have evolved through interracial hybridization among first, second phase Cytoraces along with parental races. Each of these Cytoraces is composed of recombined genomes of the parental races. Here, we have made an attempt to systematically assess the impact of hybridization on karyotypes, morphometric and life history traits in all 16 Cytoraces.

RESULTS

The results reveal that in most cases, the newly evolved Cytoraces, with different chromosome constitutions, exhibit decreased body size, better fitness and live longer than their parents. Particularly, Cytorace 5, 6 and 8 have evolved with very much higher range values of quantitative traits than the parents and other Cytoraces, which suggests the role of transgressive segregation in the evolution of these Cytoraces.

CONCLUSION

Thus, the rapid divergence recorded in the chromosomes, karyotypes, body size and fitness traits of Cytoraces exhibit the early event of recombinational raciation / speciation in the evolution of the Cytoraces under laboratory conditions.

Genetic architecture of a wing size measure in Drosophila hibisci from two populations in eastern Australia

Two models of evolutionary change invoke either additive genetic contributions to phenotypic traits (Fisher) or epistatic as well as additive effects (Wright). An earlier study of the flower-breeding Drosophila hibisci from two sites in eastern Australia reported additive and epistatic genetic effects as well as environmental effects on ovariole number. The present study of the same flies examines the genetic architecture of wing width, a trait that is correlated phenotypically with ovariole number and body size. A generation means analysis of flies reared at 25 degrees C indicated additive and epistatic genetic effects, but no consistent maternal effects, whereas for flies reared at three temperatures (18 degrees C, 21.5 degrees C, and 25 degrees C) linear and nonlinear environmental effects interacted with additive genetic effects. The genetic correlation matrix for ovariole number and wing width suggested negative genetic correlations between additive effects on one trait and epistatic effects on the other. Both traits provide evidence of genetic effects consistent with assumptions of Wright's shifting balance theory of evolution.