Despite its morphological similarity to the other species in the Drosophila melanogaster species complex, D. sechellia has evolved distinct physiological and behavioral adaptations to its host plant Morinda citrifolia, commonly known as Tahitian Noni. The odor of the ripe fruit of M. citrifolia originates from hexanoic and octanoic acid. D. sechellia is attracted to these two fatty acids, whereas the other species in the complex are repelled. Here, using interspecies hybrids between D. melanogaster deficiency mutants and D. sechellia, we showed that the Odorant-binding protein 57e (Obp57e) gene is involved in the behavioral difference between the species. D. melanogaster knock-out flies for Obp57e and Obp57d showed altered behavioral responses to hexanoic acid and octanoic acid. Furthermore, the introduction of Obp57d and Obp57e from D. simulans and D. sechellia shifted the oviposition site preference of D. melanogaster Obp57d/eKO flies to that of the original species, confirming the contribution of these genes to D. sechellia's specialization to M. citrifolia. Our finding of the genes involved in host-plant determination may lead to further understanding of mechanisms underlying taste perception, evolution of plant–herbivore interactions, and speciation.
The evolutionary genetic studies of more than ten species or subspecies belonging to the D. nasuta subgroup lead the following conclusions :Morphological differentiation has been observed qualitatively as well as quantitatively, but closely related species could not be identified by means of the techniques of external morphology.By Judging from the allozyme analyses, the local populations of D. nasuta were genetically similar in two Esterase loci. The Chiangmai population was differentiated from other allopatric populations of D. sulfurigaster albostrigata. The pattern of the speciation process in the D. nasuta subgroup was discussed.
To reveal the genetic mechanism of host selection in a monophagous fruit fly Drosophila sechellia, olfactory responses and oviposition preferences of this species were compared with those of closely related polyphagous species, D. simulans and D. melanogaster. Adult flies of D. sechellia were strongly attracted to the ripe fruit of Morinda citrifolia which is known to be the sole breeding site of this species. They were also attracted to the odor of n-caproic acid which is contained in the ripe fruit of M. citrifolia and is presumably responsible for the characteristic odor of the fruit. In contrast, D. simulans and D. melanogaster showed a strong repulsion to n-caproic acid. In parallel with the olfactory responses, D. sechellia females laid eggs preferentially on a medium containing n-caproic acid, to which the other two species showed an aversion. Genetic analyses using the hybrid progeny between D. sechellia and D. simulans suggested that the species differences in these behaviors are controlled by gene(s) located on the second chromosome.
A unique male sterile mutant of Drosophila melanogaster, ms(3)K81 (3-91.3), has been discovered. The male homozygous for ms(3)K81 produces those abnormal sperm which are defective in syngamy but are still capable of activating eggs. The eggs activated by these sperm usually produce haploid embryos that die before hatching.But some of them, by restoring diploidy, undergo normal development to produce impaternate adults. This demonstrates that D. melanogaster has a capacity to reproduce gynogenetically.
A simple olfactometer was constructed for measuring the responses of adult Drosophila to odoriferous substances, and an index (attractability index, AI) is proposed to evaluate the response. Five strains of Drosophila melanogaster of different geographic origins were tested for their responses to five chemical attractants: ethyl alcohol, acetic acid, lactic acid, ethyl acetate, and n-butyraldehyde. Statistical analysis has shown that the differences of AI among strains are highly significant for all the odorants except acetic acid and that male flies are, in general, less sensitive than females. The number of flies which were inactive in the olfactometer also varied with strain, sex, and odorant, suggesting the involvement of some other behaviors, and this raises some difficulties in interpreting the heterogeneities of AI. Nevertheless, these results suggest that a considerable amount of genetic variation for olfactory response exists in natural population of D. melanogaster.
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