Clayberg, C. D.
The sterility of a mutant in the tomato variety San Marzano results from both meiotic asynapsis and extensive chromatin degeneration in the pollen-mother-cells. In greater than ninety per cent of the microsporocytes all chromatin degenerates similarly, clumping or disappearing completely. Chromatin may also disintegrate differentially, as is suggested by the frequent occurrence of microsporocytes with less than the normal complement of chromosomes.
Chromosome behavior of the asynaptic at meiosis resembles that reported for complete asynaptics, but a low level of pairing is maintained throughout prophase and metaphase. Since most cells have no paired chromosomes, pairing is considerably less than that previously found in other asynaptic tomatoes by Soost (1951). The unpaired chromosomes do not form a plate but lie in a common spindle, which may elongate into a crescent shape, or more strongly into a U-shape or even a circle. Dividing only rarely in the first division, univalents are distributed randomly between two or more nuclei. The second division usually is normal, although the chromatin has disintegrated in most cells by this stage. Pollen abortion is complete, and ovule fertility is much reduced.
Meiosis in asynaptic buds is initiated somewhat later than in normal material.
If inheritance of the asynapsis is assumed to be determined by a single recessive gene, a marked deficiency of asynaptic segregants occurs in the F2 families. One such family, containing only one asynaptic in 29 plants, has been progeny tested. Eighteen of the progenies are segregating for asynapsis, closely fitting expectations for recessive monogenic inheritance. But a homogeneity X^2 test indicates that not all of these progenies are segregating in the same proportion. Five are deficient in asynaptic plants on the basis of a 3:1 ratio.
Inheritance of the asynapsis by two linked recessive duplicate factors has also been tested. Calculations from F2 and F3 data using the maximum likelihood method give a recombination value of 0.40 +- 0.55 units. The confidence limits of this estimate suggest that the two hypothesized genes may also be unlinked.
The information available is not sufficient to distinguish between mono- and digenic inheritance.
Limited results indicate that the asynapsis does not alter the crossing-over rate, as tested in the Wo-d region.