Chromosome studies on the mechanism of meiosis in Melanoplus femur-rubrum.

Abstract

Chromatids were traced throughout the chromosome configurations in the first meiotic division in Melanoplus femur-rubrum. Chromosome configurations were studied in relation to the different hypotheses concerning the interpretation of chiasmata and the mechanism of chromosome pairing. A ratio of 7l compensating to 35 non-compensating chiasmata was found.Cytological interference was demonstrated. Chiasma frequencies were determined and a reduction in frequency was shown to be due to terminalisation rather than to breaks at chiasmata. Terminal associations were demonstrated to arise from earlier interstitial chiasmata. Movement of chiasmata was shown to be, in some cases, towards the spindle attachment. This is discussed in relation to recent hypotheses concerning the movement of chiasmata. The interpretation of certain configurations was found impossible on the ~classical" hypothesis. In a heteromorphic bivalent, the one chiasma formed must have been the result of previous crossing-over. The greater frequency or asymmetrical arrangements of chromatids is in accord with the "partial chiasmatypy hypothesis". A greater degree of contraction of chromosomes was found at metaphase in the spermatogonial divisions than atmetaphase in the first meiotic division, which is not in accord with Darlingtont's precocity theory of meiosis

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