Chromosome fragmentation produced by crossing-over inTrillium erectum L.

Summary

A chromatid fragment lacking an attachment has been found to occur in about 12 per cent. of the pollen mother cells in two plants ofTrillium erectum L.

Crossing-over between a normalD-chromosome and one with an inverted segment comprising about one-sixteenth of the length of its longer arm appears to be the cause of thede novo origin of this fragment in meiosis.

Crossing-over in this region produces at the same time a chromatid with two attachments. In about one-fifth of the cases these are “sister” attachments and the chromatid therefore goes to one pole in the form of a loop in the heterotypic division. In about four-fifths of the cases the attachments are homologous and go to opposite poles and the chromatid therefore forms a bridge across the nucleus.

The “loop” type forms a bridge in the homoeotypic division. In whichever division it occurs the “bridge” chromatid usually breaks near the middle, though it may break at any point. Sometimes the “bridge” chromatid of the first division remains unbroken and it stretches between the daughter nuclei at the second division.

The fragment chromatid does not divide in either meiotic division, although the “tertiary” split of diakinesis is very clearly seen in it at all stages.