Population phylogeography and habitat fragmentation at island-wide and local scales in two endemic Hawaiian Drosophila: D. tanythrix and D. yooni

Habitat fragmentation due to natural and anthropogenic causes can be important in shaping the distribution of species into distinct genetic populations that diverge over time. This study examined the population divergence at both geographically broad island-wide and local elevation -gradient scales in two antopocerus Drosophila leaf-breeding species, D. tanythrix and D. yooni, which are endemic to the Island of Hawaii, Big Island. Morphological characters were used to group these two species with males of the two species determined by long hairs on the front tibia legs and females distinguished by general body color, which can provide more challenges to identify species. Population genetic differentiation was assessed through sequence variation at the Cytochrome Oxidase II (COII) gene region of the mitochondrial genome and a Y-chromosome gene of the nuclear genome. The genetic variation at the COII gene was consistent with the morphological separation of the two species in the main island-wide sites, except for some inconsistencies of female D. tanythrix with the D. yooni, which was expected. However, in the Mauna Loa Saddle sites, where there has been a recent lava flow, all of the individuals were morphologically identified as D. tanythrix, but the COII haplotypes were of both the typical D. tanythrix and D. yooni haplotypes. This could be the result of a recent hybridization between these two species with the local extinction of D. yooni in the Mauna Loa Saddle area. The nuclear Y-chromosome gene was able to distinguish the two species consistent with the morphology. Populations of D. tanythrix species found within the naturally remaining wet forest habitat indicated that South Kona and Laupahoehoe are genetically diverged from the rest of the populations at the COII gene. On the other hand, D. yooni showed lower overall genetic variation and only low levels of genetic divergence at both broad island-wide and local sites. This may indicate that D. yooni is a more recently derived species that has more recently spread around the island. The nuclear Y-chromosome gene showed that the D. tanythrix individuals collected from the Mauna Loa Saddle have a unique haplotype which is different from the broad island-wide D. tanythrix as well as D. yooni individuals. The recent habitat fragmentation in the Kipuka region of Mauna Loa Saddle region may have led to this novel Y-chromosome haplotype through a genetic founder event. Overall, the natural habitat fragmentation of the wet forests on the Island of Hawai'i appears to be an important factor isolating species into separate genetic populations and creating unique genetic populations.