Nuclear behavior of Spirostomum ambiguum during conjugation with special reference to macronuclear development.

Abstract

SYNOPSIS. During conjugation in Spirostomum ambiguum, the micronuclei divide thrice before synkaryon formation and 20 times thereafter. During the first meiotic division 18‐24 bivalents, each about 0.5 μ or less appear on the spindle. They separate and pass to the poles. The details of the 2nd and 3rd prezygotic divisions and synkaryon formation by reciprocal exchange of gametic nuclei resemble those described for other ciliates in the literature. The synkaryon divides twice resulting in 4 nuclei; 2 of them become micronuclei and the remaining 2 macronuclear anlagen. The micronuclei enter into division, but this division is arrested in metaphase. The chromosomes in the macronuclear anlagen resemble those appearing in the Ist meiotic division in shape and size. In their maximum stage of development the macronuclear chromosomes are at least 3‐4 times larger than those appearing in the arrested micronuclear metaphases in the same cell. There is no banding pattern of the chromosomes and therefore the possible extent of polyteny is difficult to evaluate. The chromosomes duplicate 3‐4 times resulting in about 200–250 before they become indistinct as separate entities. Spirostomum is the only nonhypotrichous ciliate in which these cytologic features are described.