Transmission of a deficiency for the entire nucleolus organizer region

Certain maize heterozygous interchanges with one breakpoint near the end of a chromosome will transmit through the ovules a deficiency plus duplication (Df + Dp) chromosome complement that is deficient for a short end segment. A genetic marker located in that end segment can be used to identify heterozygous Df + Dp plant types among the progeny. Previous crosses of heterozygous NOR-interchanges (interchanges with one breakpoint in the nucleolus organizer region) as female, with heterozygous polymitotic (+/po) males, produced male steriles (-/po hemizygotes) among the progeny (MGCNL 51:49-52, 1977). Those data suggested that the transmitted deficiency can include the NOR-secondary constriction, and as much as 90% of the NOR-heterochromatin. This was a surprise, since the NOR is the site of the 18S/25S rRNA genes, and some of these transmitted deficiencies presumably were deficient for at least 90% of the rRNA genes. This report provides evidence for the transmission of gametes missing the entire NOR.

Two interchanges with breakpoints known to be proximal to the NOR were crossed as heterozygous females to +/po males. These interchanges were T5-6c (5L.89, 6S near centromere) and T6-9(067-6) (6S.39, 9L.47). Smaller seed were selected to increase the frequency of putative Df + Dp heterozygotes among the resulting plants. Partial microsporocyte samples were taken from all of the progeny, in order that the same plants could later be scored for male sterility. Tests with T5-6c yielded no male-sterile progeny among over 100 plants derived from smaller seed. This result was expected because the deficiency would be for the whole short arm of chromosome 6. However, tests with T6-9(067-6) indicated the successful transmission of the Df + Dp gamete deficient for the + allele of po. Planting 16 smaller seed from each of eight crosses yielded 26 male-sterile (-/po) plants from five of the crosses. At least six fertile plants were noticed to be phenotypically heterozygous for the Df + Dp chromosome complement; these presumably received the + allele from the +/po male parent in the cross. Cytological analysis of male-sterile putative Df + Dp heterozygotes clearly indicated the presence of a normal chromosome 6 and a Df + Dp chromosome. Synapsis was normal in all cases through 900A of 6S between the centromere and the NOR-heterochromatin. Occasionally, the normal and the Df + Dp chromosome would then show asynapsis just proximal to the NOR. No indication of the NOR-heterochromatin or the NOR-secondary constriction was observed in the Df + Dp chromosome. The original report of the 6S breakpoint being at .39 probably is incorrect. These results suggest the breakpoint is proximal but near the NOR.

The hemizygous tests reported here indicate that a megaspore missing the entire NOR region can function in the postmeiotic mitoses of the developing embryo sac, and in the subsequent fertilization and sporophytic developmental processes. Previous evidence in maize shows that the NOR is the site of the 18/25S rRNA genes. The evidence reported here showing transmission of a deficiency for the entire NOR indicates that megametogenesis can proceed on the basis of maternal ribosomes carried over during embryo sac development, or that rRNA genes exist in the genome at locations other than the NOR, or both. Such numbers of rRNA genes at any one non-NOR site must be fairly low since the rRNA/DNA in situ hybridization procedures we have employed in the past allow the detection of clusters of 50 rRNA genes or more.

R. L. Phillips, A. S. Wang and W. P. Bullock


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