Drosophila silvestris is a large species of fly in the family Drosophilidae that are primarily black with yellow spots. As a rare species of fruit fly endemic to Hawaii (“the Big Island”), the fly often experiences reproductive isolation. Despite barriers in nature, D. silvestris is able to breed with D. heteroneura to create hybrid flies in the laboratory.
Male D. silvestris demonstrate many elaborate courtship displays like wing waving and courtship songs to attract females to their territories. To defend these territories, males behave aggressively and fight with one another. This species demonstrates sexual selection through female choice, as indicated by an evolutionary enhancement of extra tibia bristles occurring in certain D. silvestris populations in the last 700,000 years.
Description
D. silvestris has a large body, long legs, and slim wings. This fly has a round head shape, which is typical for fruit flies. While females have a completely dark yellow or orange face, males have a black face with a horizontal yellow stripe between the frons and the clypeus. The antennae are black in males and yellow in females. The lateral part of the mesothorax varies from brown to black pigmentation, and yellow pigmentation can be found on the edge of the top side in some individuals.
Unlike females, males have two rows of long bristles along the upper sides of their forelegs. There is some variation in the number of male tibial bristles, as 20-30 extra bristles can be found in the middle of the two rows in north and east populations of Hawaii. These extra bristles create an irregular third row, which is related to the fly's courtship ritual and may have arisen due to geographic isolation from ancestral south and west populations.
This species is tolerant to colder temperatures and vulnerable to heat stress due to higher temperatures, which affect sperm mobility and survival. This is consistent with the flies' preference for cooler environments found at higher elevations. There are three phylogenetically early chromosomal inversions in D. silvestris that are associated with elevation changes, possibly reflecting an adaptive shift to altitude that contributed to evolution in Hawaiian Drosophila.
Distribution and habitat
Currently there are about 1,000 Drosophila species native to the Hawaiian Islands, and, D. silvestris inhabits the Big Island.
Extensive lava flow from volcanoes fragments the geographical distribution of this species around the island. The flow of lava creates kipukas, isolated habitable patches of vegetated land, which can range from several square meters to several square kilometers in size. D. silvestris inhabits high-altitude, cool rainforests on the slopes of all five volcanoes in Hawaii (Mauna Kea, Mauna Loa, Kilauea, Hualalai and Kohala), specifically in altitudes ranging from . The most ancestral populations of this fly species are found on Hualalai, the third youngest volcano on the island.
Evolution and taxonomy
A quarter of the world's Drosophilidae species are endemic to the Hawaiian Islands evolving from an ancestral continental species which arrived 26 million years ago due to Hawaii's extremely diverse ecosystem and vegetation causing speciation. D. silvestris only inhabits the Island of Hawaii (the Big Island), which is 400,000 years old.
D. silvestris is a member of the "picture-winged" group, which consists of 112 Hawaiian Drosophila species known for their unique light and dark wing patterns. This group consists of four subgroups; D. silvestris is a part of the planitibia, or idiomyia, subgroup, which is known for consisting of flies that produce courtship songs by vibrating their wings. D. silvestris and other recently evolved planitibia species emit less complicated and higher frequency courtship songs compared to this subgroup's more ancestral flies.
D. heteroneura, another member of the plantibia subgroup, is a close relative of D. silvestris. Both of these fly species are native to Hawaii, and genetic similarities such as the sharing of a unique inversion in Chromosome 3 provide evidence that these fly species are derived from the same ancestral Hualalai population despite being morphologically distinct. The close evolutionary relationship between these two species is supported by a lack of this inversion in D. planitibia and D. differens, the more ancestral members of the planitibia subgroup.