Microsporogenesis and Chromosome Behavior in Nothoscordum bivalve

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Abstract
During early prophases in the pollen mother cells the strands approximate in pairs, in accord with the scheme of parasynapsis. On emerging from synizesis the spireme is clearly double and much twisted. The doubleness persists in all the later stages, the spireme becoming arranged into more or less well defined loops to form the "second contraction" figure. Transverse segmentation of the bivalent spireme shortly begins to form the bivalent pairs of chromosomes. The loops are not significant in this process. Constrictions other than those concerned with the segmentation of the spireme into bivalents become evident in the chromosomes at about this time, resulting in bivalent pairs of unusual form such as closed "rings" of what appear to be 4 univalent chromosomes joined end-to-end; also what seem to be 2 bivalent pairs attached end-to-end. There are in each pollen mother cell nucleus 7 such "rings" or "double pairs" and 2 bivalents of the more usual form. The apparent compound groups are really bivalents, each a pair of univalent homologous chromosomes deeply constricted in the median or submedian region. Heterotypic division is regular and reductional. No evidence of a longitudinal split was seen until mid-anaphase. There is little lagging of chromosomes. Subsequent stages, including division of the microspore nucleus, are described in detail. Of the 9 chromosomes of the haploid complement, 7 have median or submedian constrictions, the other 2 show small subterminal constrictions or satellites. Root-tip nuclei show 18 chromosomes with the same types of constrictions found in dividing pollen mother cell and microspore nuclei. Two hypotheses are suggested for the possible origin of Nothoscordum from Allium.

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