THE STRUCTURE OF MEIOTIC CHROMOSOMES IN THE GRASSHOPPER AND ITS BEARING ON THE NATURE OF "CHROMOMERES" AND "LAMP-BRUSH CHROMOSOMES"

Abstract

The structure of meiotic chromosomes in the grasshopper was studied to re-examine the evidence for the reality of "chromomeres"". It was found that the "chromomeres" in leptotene chromosomes are actually turns of a tight helix. This coil is already present in the unravelling chromosome of preleptotene. The zygotene pairing occurs between 2 coiled chromosomes, the gyres fitting into each other so that the bivalent forms a paranemic spiral. During prophase the helix increases in width and decreases in the number of gyres to become the major coil of diakinesis and metaphase. The minor coil, first visible in late pachytene, consists more of an irregular twist in the chromonema than a regular helix. Beginning in late pachytene the gyres of the chromatid major coils separate laterally so that the chromosome assumes a loose, reticulate structure. This corresponds to the "diffuse stage" observed in the diplotene of many insects. It is suggested that the vacuolated appearance of many chromosomes is due to this lack of appression of component chromonemata. In diakinesis the independence of the major coils of chromatids persists. The fuzzy appearance of these chromosomes is caused by the peripheral projection of the loops of chromonemata. "Lamp-brush chromosomes" in the oocyte of many vertebrates have the same appearance as diakinesis chromosomes in the grasshopper, except for the tremendous increase in length. It is here suggested that the observed loops or "hairs" in these chromosomes are but the gyres in the major coil of the laterally separated chromonemata. The fundamental unit of the chromosome is a microscopically uniform thread, variable in length depending on the metabolic state of the nucleus. The differential coiling of this chromonema leads to the longitudinal differentiation of the chromosome. What had been described as "chromomere" are misinterpretations of gyres of the chromonematic helix (leptotene, somatic prophase), or points of overlap of chromonemata (diplotene), or heterochromatic sections consisting of tightly coiled regions of the chromonema.