Evolutionary Genetics Lab.Research
Significance
Have you heard about interspecific hybrids? Mules from mares and donkeys, leopons from lionesses and leopards… Interspecific hybrids are rare in the nature and most of them are sterile, thus cannot produce descendants. If they were not rare, biological species would become fused and lost from the world. In other words, species exist because of reproductive isolation. The origin of new species is completed by acquiring reproductive isolation between populations. Therefore, speciation is a major driving force of evolution. The purpose of our research is to elucidate the genetic mechanisms of speciation.
In the possession of National Museum of Nature and Sciences, Tokyo (Courtesy of Dr. S.-i. Kawada). Reproduced, with permission, from Sawamura et al. (2011) Iden Vol. 65, No. 3, pp. 2-4.
Materials
We use Drosophila. The genomic sequencing has been completed in 12 species and the sequenced species will soon be 30. Their biodiversity is also spectacular: 3950 extant species (and 12 fossil species) have been described in Drosophilidae (World Catalogue of Insects Vol. 9). Furthermore, there are many crossable sibling species pairs, and Drosophila can provide useful experimental systems to investigate genetic mechanisms of speciation.
We especially use two species complexes belonging to the melanogaster species group. One is the melanogaster complex originating from Africa and the Indian Ocean (Madagascar, Mauritius, Seychelles); many evolutionary biologists all over the word use them as the model of speciation. The other is the ananassae complex originating from Southeast Asia and the South Pacific (Papua New Guinea, Samoa, Tonga, Fiji, northern Australia); they are difficult to be distinguished by appearance but can be separated by crossing experiments. Three of them have been described but there are more unnamed cryptic species. We use the materials collected from those places mentioned above.
Postmating isolation
The cross between D. melanogaster females and its sibling species (e.g., D. simulans) males produces only sterile hybrid females; hybrid males die at the larval stage. In the reciprocal cross most of the hybrids produced are males; hybrid females die at the embryonic stage. The genetic bases of postmating isolation have been elucidated by analyzing mutations that rescue the hybrids from lethality or sterility.
The genes isolated so far are the followings: (1) Lethal hybrid rescue encodes a heterochromatin binding protein and is involved in hybrid male lethality, (2) Hybrid male rescue encodes a chromatin binding protein and is involved in hybrid male lethality and female sterility, (3) zygotic hybrid rescue represents satellite DNAs (repetitive sequences) consisting of heterochromatin and is involved in hybrid female lethality, (4) Nucleoporin 160 encodes a nuclear pore complex protein and is involved in hybrid male lethality and female sterility, (5) Nucleoporin 96 encodes a nuclear pore complex protein and is involved in hybrid male lethality, (6) JYalpha is located on different chromosomes between the species and leads to sterility of hybrid males not carrying the gene.
Premating isolation
Males of D. ananassae and D. pallidosa emit different courtship songs and the interspecific cross is difficult. But once the sexual isolation is overcome, fertile hybrid females and males are produced, thus producing descendants. By using a special technique (hybrid parthenogenesis), we have made interspecific mosaic genome lines (homozygous clones). And we investigated each line whether the females accept D. ananassae males or D. pallidosa males. The left arm of chromosome X, the left arm of chromosome 2, and the right arm of chromosome 3 are involved in the female behavior. Genes responsible for reproductive isolation might be easily accumulated in such genomic regions where species-specific inversions are present. Molecular phylogenetic analyses also support this idea: interspecific gene flow has been detected in collinear genomic regions. This is consistent with the recently advocated “chromosomal speciation” model in sympatric populations.
