DNA synthesis patterns in the giant foot-pad nuclei of Sarcophaga bullata (Sarcophagidae, Diptera)

Endopolyploidy as a potential driver of animal ecology and evolution

Endopolyploidy - the existence of higher-ploidy cells within organisms that are otherwise of a lower ploidy level (generally diploid) - was discovered decades ago, but remains poorly studied relative to other genomic phenomena, especially in animals. Our synthetic review suggests that endopolyploidy is more common in animals than often recognized and probably influences a number of fitness-related and ecologically important traits. In particular, we argue that endopolyploidy is likely to play a central role in key traits such as gene expression, body and cell size, and growth rate, and in a variety of cell types, including those responsible for tissue regeneration, nutrient storage, and inducible anti-predator defences. We also summarize evidence for intraspecific genetic variation in endopolyploid levels and make the case that the existence of this variation suggests that endopolyploid levels are likely to be heritable and thus a potential target for natural selection. We then discuss why, in light of evident benefits of endopolyploidy, animals remain primarily diploid. We conclude by highlighting key areas for future research such as comprehensive evaluation of the heritability of endopolyploidy and the adaptive scope of endopolyploid-related traits, the extent to which endopolyploid induction incurs costs, and characterization of the relationships between environmental variability and endopolyploid levels.

Influence of age and diet on DNA synthesis and the DNA content of mid-gut cells in the dipteran Sarcophaga carnaria

Virgin female Sarcophaga carnaria maintained on a diet of sugar and water had shorter lifespans than those maintained on a liver, sugar and water diet; there was no significant different between the lifespans of mated females or mated males kept on the two diets. Autoradiographs of mid-gut sections from virgin females which had been fed with [3H]thymidine showed substantial amounts of DNA synthesis in the nuclei during at least the first 14-15 days of adult life. Cytophotometric measurements of Feulgen-stained mid-gut nuclei of 0--1-, 2--3-, and 14--15-day-old females showed that the integrated absorbances peaked at values corresponding approximately to 2 C, 4 C, 8 C, 16 C and 32 C. The proportion of nuclei with the higher Feulgen-DNA values increased with age, and was greater in flies maintained on the liver, sugar and water diet. It is suggested that the DNA synthesis observed is associated with polytenization rather than mitotic cell division, and that the level of polyteny reached may reflect age- and diet-related differences in the functional demand made on mid-gut cells.

Characterization of extrachromosomal DNA in the flesh fly Sarcophaga bullata

The polytene pupal foot pad cells of the flesh fly Sarcophaga bullata contain numerous extrachromosomal DNA containing granules. We have determined both the origin and the nature of the DNA sequences present in these granules. Studies done with quinacrine staining of seven day old pupal foot-pad polytene nuclei showed that the granules fluoresced very brightly while the chromosomal bands to which the granules were attached did not. The only other highly fluorescent regions of the polytene karyotype were the centromeric heterochromatin of chromosomes C and E and several bands associated with the nucleolus of Chromsome A. When polytene nuclei were hybridized in situ with cRNA made from highly repetitive DNA, many of the granules positively labeled. Most of the label on these slides was concentrated on the centromeric heterochromatin of chromosomes C and E. Quinacrine staining of the foot-pad cells at very early stages of pupal development showed that when granules were present, they were always closely associated with the same two centromeric regions, those of chromosomes C and E. Since the highly repetitive DNA located in these centromeric regions is underreplicated, we conclude that the granules result from an extrusion process which takes place early during the polytenization of these cells. The chromosomal integrity of the centromeric heterochromatin of chromosomes C and E is apparently disrupted and repetitive sequences are dissociated from the chromosomes as DNA granules which then secondarily become associated with chromosomal bands throughout the nucleus.

Patterns of incorporation of tritiated thymidine by the dorsal polytene foot-pad nuclei of Sarcophaga bullata (Sarcophagidae:Diptera)

The incorporation of tritiated thymidine into the DNA of the dorsal foot-pad nuclei of Sarcophaga bullata, during pharate adult development, was studied by scintillation counting and autoradiography. Incorporation was maximal on day 4 and showed a progressive temporal decline on days 5 to 8. Autoradiographs of chromosomal arms A1, A2 and D1, from late stages of the prolonged S period, showed discontinuous incorporation. On days 7 and 8 loci which incorporated the radio-labeled precursor were almost exclusively associated with DNA granules. DNA granules were associated with 8 specific loci in chromosomal arm A1, 10 in A2, and 8 in D1. The relationship between patterns of incorporation of tritiated thymidine and DNA granules is discussed.