Studies on Genetic Mechanisms Controlling Univalent Transmission in Wheat

I. A comparison of two pentaploid hybrids having natural and synthetic common wheat as the hexaploid parent

Abstract

The pentaploid hybrid, abbreviated N-5x (5x=2n=35, genome formula AABBD), between a natural common wheat, Triticum spelta var. duhamelianum (6x=2n=42, AABBDD) and an emmer wheat, T. durum var. melanopus (4x=2n=28, AABB) transmitted 2.64 D-chromosomes (chromosome belonging to the D genome) per fertilized female gamete. However, the pentaploid hybrid (S-5x) between a synthetic common wheat (amphiploid between T. durum cv. Golden Ball and Aegilops squarrosa; 6x=2n=42, AABBDD) and the same emmer wheat transmitted 1.23 D-chromosomes per fertilized female gamete. Both transmission rates were significantly lower than the theoretically expected rate (3.5 chromosomes/gamete).
Cytological events occurring in meiosis in PMCs were investigated in detail in both pentaploid hybrids. In N-5x, 7.6 univalents per PMC were found at MI per PMC, and 67% of them formed lagging chromosomes at At (10.2/PMC). Most of these lagging chromosomes reappeared as laggards at All (9.76/tetrad), and about 40% of them eventually formed micronuclei at TII (3.95/tetrad). Only 6.1% of the laggards at At formed micronuclei at TI. Elimination of univalents from the nucleus, first as laggards and finally as micronuclei, fully explains the transmission rate of D-chromosomes in N-5x. In S-5x, however, 8.3 univalents per PMC were found at MI, and 46.4% of them formed laggards at At (7.7/PMC). Again, most appeared as laggards at All (6.82/tetrad), and about 38% of them formed micronuclei at TII (2.58/tetrad). The portion of At laggards forming micronuclei at TI was only 8.2%. In this pentaploid, univalent elimination should result in the transmission of 2.85 D-chromosomes/gametes. Reduction of the D-chromosome number by univalent elimination explains only 30% of the actual observed reduction. In S-5x, preferential fertilization of 14 and 15-chromosome gametes seems to play a more important role than does univalent elimination in determining the transmission rate of D-chromosomes.
Finally, the segregation of transmission patterns of univalents (N-5x type vs S-5x type) was studied with four pentaploid hybrids recovered in the Bl generation, N-5x F₁×emmer parent. Each of two B₁ plants showed an N and S-5x type transmission pattern, while the remaining two were intermediate to the two F₁ types. Since both parental types were recovered in a small B₁ population, the transmission pattern of univalents seems to be controlled by major genes. However, they must number two or more, because the intermediate type also appeared in the B₁ generation. The origin and evolutionary role of the genes controlling a high transmission rate of univalents were discussed.