The Scaptodrosophila, an Australian drosophilid genus
Ann J. Stocker
Scaptodrosophila are a diverse group of flies whose appearance is very similar to the widespread and widely studied Drosophila. The genus is estimated to have diverged within the drosophilid lineage during the Cretaceous period about 70 million years ago. It was originally considered a subgenus of Drosophila and called Pholadoris in earlier studies but a taxonomic revision of the family elevated it to genus rank.
Scaptodrosophila are very prevalent in Australia, representing about a third of the described drosophilid species, but have received limited research attention. The developmental requirements of most Scaptodrosophila species are unknown and appear to be restrictive since few species are widespread in urban environments. Scaptodrosophila species have been reported as feeding or breeding on tree sap, fungi, fruit and flowers. Unlike Drosophila species, most cannot be collected on fruit baits, except for members of the coracina species group. Scaptodrosophila hibisci and S. aclinata breed in the flowers of native Hibiscus species. There are also species that feed on and form galls in the stems of a bracken fern, while others feed on eucalyptus leaf litter in the crowns of tree ferns. Species from the genus may have important ecological functions such as in pollination and nutrient cycling, as well as in acting as a food source for other organisms.
They are not easy to maintain in the laboratory since, beside specific food requirements for some species, they do not pupate within their food vial. Instead, they launch themselves over the side and burrow into surrounding soil or vermiculite, only emerging as adults. This means that each generation must be “sucked” up, fly by fly, and placed into new food vials. All larvae that we have examined so far have a skipping movement which seems to involve contracting and ‘snapping’ their body.
A number of Scaptodrosophila were brought into our laboratory as part of a large sequencing study that we were involved in. They were identified by a member of our group, Michele Schiffer, and cared for by a very competent technician (Jennifer Shirriffs). At the end of this study, the flies were to be sent elsewhere. I felt that here was an opportunity to have a look at their karyotypes before they disappeared. To my knowledge only one such study had been done – by Ian Bock on six members of the lativittata complex of the coracina group. I did not see my work as an independent publication, but perhaps as an addition to the ongoing sequencing study. As it turned out, it became an independent study done in two countries.
I managed to examine 12 species, several that had so far not been described. This showed that despite their long period of separation, Drosophila and Scaptodrosophila were chromosomally very similar. Although no species that we looked at had the ancestral Drosophila karyotype of 5 acrocentric (rod) chromosome pairs and 1 pair of “dots”, the Scaptodrosophila karyotypes observed could have been derived from this ancestral karyotype by arm changes that are common in Drosophila. Location of heterochromatin was examined by C-banding and was found to be similar to that seen in Drosophila species. In particular, the Y and “dot” chromosomes were heterochromatic, as were centromere-associated regions and an X chromosome arm.
We also examined the salivary gland polytene chromosomes and found numerous underreplicated regions within their arms which should be heterochromatic but did not consistently C-band in ganglion cells. There were also inverted repeat regions. Such regions caused twists and adhesions, often preventing the arms from spreading cleanly.
In Drosophila, sequences producing rRNA and forming a nucleolus are usually within heterochromatin regions on the sex chromosomes. We used in situ hybridization to locate these regions in some of our Scaptodrosophila species. In most of the species we examined there was an rDNA site on the X and the Y chromosome as expected. However, in Scaptodrosophila sp. aff. concolor strain CBN17 the rDNA site was on the “dot” chromosome, apparently toward one end. Some Drosophila species also have rDNA on the dot chromosome so this was not unusual. All the sequences producing rRNA are near heterochromatin, and association of heterochromatin regions during early developmental divisions would facilitate chromosome to chromosome exchanges. Scaptodrosophila sp. aff. concolor strain CBN17 has a submetacentric X chromosome with very little heterochromatin, which would suggest that some kind of historical exchange had occurred.
It has been interesting to have a look at this genus which diverged within the drosophilid lineage so long ago but still retains such strong similarity to its sister genus, Drosophila. Perhaps such similarities are maintained by development of flies within similar niches and the adoption of only those conservative changes that allowed better survival in those niches.
The complete article can be read in: Chromosome Comparisons of Australian Scaptodrosophila Species. Ann Jacob Stocker, Michele Schiffer, Eduardo Gorab and Ary Hoffmann Insects 2022, 13(4), 364; doi:10.3390/insects13040364