Polyteny and the functional significance of the polytene cell cycle.

Polyteny is a special nuclear differentiation reported in larval and adult Diptera, Collembola, Protista and angiosperm ovular nuclei. In not all cases, however, do discrete giant chromosomes represent homologous pairs as in the salivary gland nuclei of Drosophila or Chironomus larvae, and in Calyptratae (e.g. Calliphora) a degree of homologue separation and dissociation of the constituents results in a reticular chromosomal organization within which banded polytene regions are prominent. The correlation previously shown between the polytene cycle of replication without mitosis and the growth habit of larval Calliphora suggests that the significance of the polytene phenomenon may lie, not in a special physiology of discrete giant chromosomes, but in the cell cycle responsible for the polytene condition; chromosome morphology may not be functionally significant. The significance of polyteny versus endopolyploidy is considered in terms of the cell cycle. Whereas the endopolyploid cycle is endonuclear and thus non-disruptive of cytoplasmic syntheses, the polytene cycle further departs from a normal mitotic cycle in that there is no M-phase: the nucleus is in permanent' interphase', and clearly the cycle should not be referred to as 'endomitotic'. The physiology of polytene chromosomes is discussed: there is no evidence that the intense activities of hypertrophied salivary cells in Diptera depend on continuous monitoring at transcription level. The aceticorcein squash technique has been used to describe the chromosomal organization in two cell types known to hypertrophy without sign of an Af-phase: oenocytes of the locust Schistocerca gregaria, and the giant trophoblast cells of mouse concepti. In both cases, there is cytological evidence of polytene lateral pairing over short regions. In Schistocerca oenocytes, this organization is similar to the reticular fibrillation of polytene chromosomes seen in calyptrate epidermal and salivary nuclei. The functional significance of the polytene cell cycle is discussed in the light of its known incidence among eukaryotes, and the use of terminology examined.