I. Research Notes

Effect of chromosomes XII and XVI on the action of Neatby's virescent

E. R. SEARS

U. S. Department of Agriculture, Columbia, Missouri, U. S. A.

Plants monosomic for chromosome XVI and heterozygous for Neatby's virescent v, a largely recessive gene located on chromosome III, bear 15-20% shriveled seeds, and all their virescent offspring are disomic. This indicates that the combination mono-XVI vv is a seed lethal. Mono-XII Vv also produces shriveled seeds, but in lower frequency; and about half the virescent offspring are monosomic. These monosomics are much smaller and less vigorous than their disomic sibs, showing that mono-XII vv results in poor viability when not lethal.

One virescent offspring of mono-XVI Vv had a small white sector. Instead of two normal XVI chromosomes, it carried one normal and one isochromosome. Since isochromosomes are sometimes lost somatically, it is reasonable to assume that the white sector was deficient for this chromosome and hence was mono-XVI vv. From seeds borne on the green portion of the plant, offspring were obtained which had the following doses of the arm of XVI involved in the isochromosome: two (two normal chromosomes), three (one normal and one iso), four (two isos), and five (one normal and two isos). Plants with two and three doses were virescent, with the latter being more vigorous. Individuals with four and five doses were non-virescent (i. e., green).

It appears, then that chromosomes XII and XVI, which are homoeologous with III, carry genes which compete with v. This is in line with the suggestion previously made (Sears, WIS No. 3) that V is a member of a triplicate series concerned with chlorophyll development.

(Received Nov. 5, 1957)