Fluorescent in situ hybridization in plant polytene chromosomes

Polytene chromosomes are found in specialized tissues, with high metabolic activity, of a few angiosperm genera. They differ from Diptera polytenics in several aspects, mainly because their chromatids on each chromosome are not tightly paired, nor are they so highly endoreplicated as those of Diptera. In situ hybridization with isotopic and non-isotopic probes has been successfully used in plant polytene chromosomes, mainly in Phaseolus coccineus and Vigna unguiculata, where they have been best investigated. The results reported for mitotic and polytene chromosomes of these species, and a few others, are compared aiming to ascertain the efficiency and limitations of FISH in plant polytenics. In general, polytene chromosomes either from embryo suspensor cells of P. coccineus or from anther tapetal cells of V. unguiculata proved to be quite a suitable system for localizing DNA sequences by FISH. The partially unsynapsed chromatids, typically found in plant polytenics, seem to be the most important hindrance for a precise chromosome mapping. On the other hand, the interphase polytene nucleus is a valuable system for localizing FISH signals since they conserve a spatial organization similar to that of mitotic interphase and produce much amplified signals.