Polyploidy and Nuclear Fusion in the Fat Body of Rhodnius(Hemiptera)

Abstract

The diploid number in Rhodnius is 22 in both sexes. At hatching the fat-body cells are tetraploid, with a few octoploid. This polyploidy is presumed to arise by endomitosis in the egg. In the fully nourished insect this state persists throughout life; and occasional 16n and 32n nuclei occur. Whether these arise by endomitosis or by nuclear fusion has not been established. In extreme starvation (e.g. 8 months from the time of moulting in the 4th-stage larva at 26 °C) polyploidy increases greatly. This results from nuclear fusion in interphase. The evidence is as follows: (i) All intermediate stages in fusion can be observed. (ii) Polyploidy develops many months after growth and mitosis have been arrested. (iii) It occurs mainly in those regions where the food reserves first become exhausted. (iv) When these starved insects are fed the polyploid cells divide and chromosome counts include 12n, 20n, 24n, etc., as well as the regular series 4n, 8n, 16n, 32n, etc. Contiguous nuclei which have not fused in interphase often amalgamate their chromosomes to form a single plate and spindle at metaphase and likewise produce more highly polyploid daughter cells. The high incidence of nuclear fusion in Rhodnius is ascribed to the low rate of metabolism which permits prolonged survival in the starving insect.