Paul A. Roberts

Evolution of DNA in heterochromatin: the Drosophila melanogaster sibling species subgroup as a resource.

The Drosophila melanogasterspecies subgroup is a closely-knit collection of eight sibling species whose relationships are well defined. These species are too close for most evolutionary studies of euchromatic genes but are ideal to investigate the major changes that occur to DNA in heterochromatin over short periods during evolution. For example, it is not known whether the locations of genes in heterochromatin are conserved over this time. The 18S and 28S ribosomal RNA genes can be considered as genuine heterochromatic genes. In D. melanogasterthe rRNA genes are located at two sites, one each on the X and Y chromosome. In the other seven sibling species, rRNA genes are also located on the sex chromosomes but the positions often vary significantly, particularly on the Y. Furthermore, rDNA has been lost from the Y chromosome of both D. simulansand D. sechellia, presumably after separation of the line leading to present-day D. mauritiana.We conclude that changes to chromosomal position and copy number of rDNA arrays occur over much shorter evolutionary timespans than previously thought. In these respects the rDNA behaves more like the tandemly repeated satellite DNAs than euchromatic genes.

Structure and activity of salivary gland chromosomes of Drosophila gibberosa

In Drosophila gibberosa the maximum secretory output of the salivary glands is in the prepupa rather than in the late third-instar larva. Using salivary chromosome maps provided here we have followed puff patterns from late second-instar larvae through the time of histolysis of the salivary glands 28–32 h after pupariation and find low puff activity correlated with low secretory activity throughout much of the third larval instar. Ecdysteroid-sensitive puffs were not observed at the second larval molt but do appear prior to pupariation initiating an intense cycle of gene activity. The second cycle of ecdysteroid-induced gene activity a day later, at the time of pupation, appears somewhat damped, especially for late puffs. Salivary chromosome maps provided here may also be used to identify homologous loci in fat body, Malpighian, and midgut chromosomes.

Can mutagenesis reveal major genes affecting senescence

The Drosophila melanogaster genome was screened for sex-linked recessive and autosomal dominant mutations affecting the senescent decline in motor abilities. No persistent identifiable genes delaying the onset of senescence were recovered among 18,089 F1 male offspring of males exposed to radiation or the chemical mutagen, EMS.

Paracentric inversion polymorphism in the grasshopper Boonacris alticola

Paracentric inversion heterozygosity can be detected at pachytene by observation of frequent regions of asynapsis and reinforced by observation of the elimination of a chiasma in the region of the inversion at diplotene and by a low level of bridges and fragments at anaphase. We present evidence that paracentric inversion polymorphism can persist in a grasshopper population despite a low level of crossing over within the inverted region in heterozygotes. Lethality resulting from aneuploidy due to limited crossing over within the region of the inversion appears to be more than compensated for by heterosis.

Synthesis and secretion of salivary gland proteins in Drosophila gibberosa during larval and prepupal development

SummaryThe late larvae of Drosophila gibberosa Patterson and Mainland choose different pupariation sites than the larvae of Drosophila melanogaster Meigen. Since the larvae of D. gibberosa do not attach themselves to the substratum, the salivary glands contain only a small amount of the “glue” proteins before pupariation. Proteins comprising the salivary gland secretions of late larvae of these two species were compared and found to be qualitatively quite different. Only five polypeptides with the same molecular masses were identified in both species. The rate of protein synthesis in the salivary glands of D. gibberosa continued to increase through the late larval stage and pupariation. As a consequence, the total amount of protein contained in the salivary glands also continued to increase after pupariation. To demonstrate temporal changes in protein synthesis from 48 h before pupariation to 28 h after pupariation, newly synthesized polypeptides were pulse labeled by culturing salivary glands in vitro. The patterns of polypeptide synthesis fell into four major groups depending upon whether the synthesis of a protein stopped shortly after pupariation, stopped during late pupariation, increased at pupariation, or was initiated after pupariation. Changing patterns of protein synthesis are correlated with the known changes in gene puffing during this developmental period.

Developmental changes in midgut chromosomes of Drosophila gibberosa

In Drosophila gibberosa, differences between midgut and salivary gland chromosomes fall into two categories: tissue-specific band modulations which persist throughout the 90 h developmental period that we studied and tissue-specific puffs. Puffs that are common to both tissues tend to appear earlier in the midgut. Some major early ecdysteroid-induced puffs appear simultaneously in both tissues at the end of the third larval instar; however, the many late puffs that follow in the salivary glands are absent from the midgut. Intense puff activity in the early third larval instar midgut declines at the time of the hormonal pulse that initiates intense gene and secretory activity in salivary glands; the sloughing of midgut cells ensues.

The consequences of fat body transplantation into young and old Drosophila

Despite some age-related changes in cells, we observed no massive fat body involution during senescence of two species of Drosophila. Transplants of larval fat failed to prolong the life of old D. gibberosa. Transplanted larval fat persisted for weeks and underwent similar changes (except for glycogen stores) in old as in young hosts, suggesting that only minor changes in hemolymph composition occur with age.

An overview of gene activity in the fat body of Drosophila gibberosa

In the larval fat body of Drosophila gibberosa, polytene chromosome structure and activity exhibit cytological differences from chromosomes of midgut and salivary glands. These differences include long-persisting puffs, transient puffs and long-persisting band modulations. Some early ecdysteroid-induced puffs are present in all three organs but few late puffs are present in the fat body. Comparative studies reveal, therefore, that late larval-early pupal puffing is enhanced in salivary glands relative to gut, fat body and Malpighian tubules. After the fat body breaks up in the prepupa, the rate of programmed cell death and the corresponding slow decline of chromosomal activity also differ from cell to cell and from other organs.