Chromosome basis of small seed effect associated with r-x1

Some kernels (about 3 to 8%, depending upon the genetic background) on r-x1 ears are smaller than the normal ones. They are variable in size (ranging from about one-tenth to three-fourths of the normal kernels), well filled and viable. Since it is well documented that seed size reduction in many cases results from an aneuploid chromosome constitution in the endosperm, it is of interest to see whether the same rule applies to these cases.

A selection of 25 small kernels, from one-third to nearly one-half the size of the normals, were germinated along with 26 large ones, and chromosome counts were obtained as follows:
 
 
Seed type Chromosome number Frequency
large 20 25
21 0
19 1
26
small 20 13
21 4
19 5
22

25 of the large kernels had 20 chromosomes, and one had 19. The rate of monosomy was 4.76%. The monosomy rate reported by Weber (1983) is 9.91%. Under the assumption that the aneuploid state of half of the 10 maize chromosomes can cause seed size reduction, the present rate is reasonably consistent with the previous one. No trisome was found in this group.

The chromosome constitution is different in the small kernel group. 4 out of 22 small kernels (18%) were trisomic and 5 others (22%) were monosomic. The frequency of aneuploidy for this group is 41%, which is nearly ten times higher than that of the large kernel group. Although the chromosome constitution of the associated endosperms can not be determined for the time being, it is at least very probable that they are also aneuploid. Results of this study suggest that seed size reduction in these cases is similar to those reported by Beckett (Can. J. Genet. Cytol. 25:346-353, 1983) and Lin (Genetics 100:475-486, 1982), where an aneuploid chromosome constitution in the endosperm results in a reduction of seed size.

Bor-yaw Lin


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