Primary Trisomics of Ryegrass: Their Isolation and Identification 1

Trlsomics (2n + 1 = 15) of perennial ryegrass ( Lolium perenne L.) were obtained either by crossing clones of a near‐triploid (2n = 3x + 1 = 22) with diploid plant or open‐pollination with diploid and tetraploid plants. Trisomics occurred with a frequency of 22% on crossing with the diploid and 8% on open‐pollination. Of the trisomics isolated, 16 were primary and one a tertiary trisomic. Trisomes were identified as nucleolar (tri 2, 3, and 4) or non‐nucleolar (tri 1, 5, 6, and 7) from the association of trivalents with the nucleolus at diakinesis, and later identified from the relative size of univalents at diakinesis, mean meiotic association at metaphase I, and pollen fertility. Trisome identity in a few cases was confirmed from somatic karyotypes. The trisomics could be divided into seven groups, probably representing all the seven primary trisomes. The relatively larger trisomes, i.e., tri 1 to 4, showed a higher trivalent frequency than the smaller trisomes, tri 5 to 7, which formed mostly univalents. The trisomic for chromosome 1 was completely male‐sterile. Pollen fertility of tri 2 to 6 ranged from 4 to 80%. In general, trisomics had narrower leaves, shorter spikes, and were less vigorous during winter than their disomic sibs. The trisomic for chromosome 1, however, was highly vigorous and had dark green, wide leaves. The tertiary trisomic involved one nucleolar chromosome, was desynaptic and male‐sterile.