Genetic and cytogenetic analyses of the A genome of Triticum monococcum. III. Cytology, breeding behavior, fertility, and morphology of autotriploids

Abstract

Mature triploid seed from reciprocal (2n = 4x × 2n = 2x) crosses in Triticum monococcum was minute and shrivelled because of endosperm collapse and therefore failed to germinate. This necessitated the excision of embryos from successful pollinations and their growth in vitro to ensure subsequent germination so as to obtain viable and vigorous autotriploids. A comparison of these triploids with their diploid and tetraploid progenitors revealed that cell size, kernel weight, and pistil size increased with an increase in ploidy level. However, unlike other species, optimum expression was observed in these triploids for plant height, tillering, size of spikes, number of spikelets/spike, and leaf size. Earliness, althoughenhanced in tetraploids relative to diploids, was delayed in the triploids. Mean numbers of univalents, bivalents, and trivalents per microsporocyte were 2.65, 2.60, and 4.38, respectively. Only chains (93.5%), which formed V-shaped metaphase I (MI) configurations, frying pan (5.0%), and Y-shaped (1.5%) trivalent associations occurred. On the average, two reciprocal exchanges occurred per bivalent and trivalent. Trivalents corriented randomly at MI. At anaphase I, all sets of three homologues segreated randomly to the two poles, lagging univalents always divided equationally, and only meiocytes with such chromosomes formed micronuclei. The reasons for similarities and differences in meiotic behaviour of T. monococcum triploids with those of other species are discussed. Confirmation of the conclusions drawn with respect to the cytology of the triploids was obtained from similar cytological observations with primary single trisomics. These triploids produced euploids, primary single trisomics as well as some double and triple trisomics all of which differed phenotypically from diploids. Triticum monococcum, like most diploid species, is highly intolerant of aneuploidy. Possible reasons for the differences in levels of tolerance of aneuploidy in species like T. monococcum and those like Petunia hybrida, which are highly tolerant of aneuploidy, are discussed. Pollen fertility was high and seed fertility was very low. Reasons for the latter as well as the high fertility in species that are highly tolerant of aneuploidy and allotriploids are discussed.Key words: Triticum monococcum, autotriploid, trisomic, cytology, breeding behavior, fertility, morphology.