Molecular and developmental cytogenetics of gene multiplicity in maize : maize, cytogenetics, ribosomal RNA, endosperm, gametes

Abstract

Gene multiplicity alterations may be an important feature of maize developmental biology. A developmentally-phased change in DNA level has been found to be a striking feature of maize endosperm growth. I n the central portion of the endosperm tissue, an average DNA level of 4.5C during the first 10days post-pollination increases to an average of 90C by day 16.The ribosomal DNA (rDNA) proportion increases by 50 percent from day 12 to day 16 and returns to near initial levels by day 20. We propose that the rDNA replicates more rapidly than the majority of the non-rDNA regions between 12 and 1 6 days post pollination; subsequently the rDNA proportion decreases due to proportionately higher replication of other DNA. Various experimentally-induced modulations in ribosomal RNA gene multiplicity were achieved using 25 chromosomal interchanges each with a break in the nucleolus organizer region (NOR). Gametes which apparently have retained only 1-2 percent of the rDNA region were transmissible through the ovules. This finding raises the question whether gametes would be viable with nor RNA genes at the NOR. Molecular cytogenetic analyses lend support to the concept that maize development may be controlled in part by the multiplicity of specific genes.