Significant Variation in Number of Genes in the NBS-encoding Gene Family among Inbred Lines and in a Segregating Population of Maize, Zea mays L

A significant finding of modern genomics research is that most genes in a higher organism exist in a form of multiple gene families or multiple copies. Our previous studies showed that the number of genes in a gene family such as the nucleotide-binding site (NBS)-encoding gene family varies dramatically within a species; and the gene number variation and evolution of the gene family is shaped by selection and associated with host plant genetic variation and evolution (Zhang et al. 2010, NAR 38:6513). Here, we report that there is also a significant variation in number of genes in the gene family among inbred lines in maize. More interestingly, we found that when two inbred lines (e.g., B73 and Mo17) having a similar number of NBS genes were crossed, F₁ offspring had a number of NBS genes similar to those of the parental inbreds. However, when the offspring proceeded to F₂, the number of NBS genes varied, either loss or gain, by a few-fold among F₂ plants. At F₁₀, although the numbers of NBS genes has changed slightly among recombinant inbred lines (RILs), the number variation magnitude of NBS genes among the RILs remained the same as that of F₂ population. These results suggest that the number of NBS genes varies significantly in the segregating population of a cross and it largely occurs at early generation, especially at the F₂ generation. This finding has indicated that a significant genome balancing process occurs after two different genomes are brought into a single cell.