Rice is a leading grain crop and the staple food for over half of the "world" population. Rice is also an ideal species for genetic and biological studies of cereal crops and other monocotyledonous plants because of its small genome and well developed genetic system. To facilitate rice genome analysis leading to physical mapping, the identification of molecular markers closely linked to economic traits, and map-based cloning, we have constructed two rice bacterial artificial chromosome (BAC) libraries from the parents of a permanent mapping population (Lemont and Teqing) consisting of 400 F9 recombinant inbred lines (RILs). Lemont Cjaponica) and Teqing (indica) represent the two major genomes of cultivated rice, both are leading commercial varieties and widely used germplasm in rice breeding programs. The Lemont library contains 7296 clones with an average insert size of 150 kb, which represents 2.6 rice haploid genome equivalents. The Teqing library contains 14208 clones with an average insert size of 130 kb, which represents 4.4 rice haploid genome equivalents. Three single-copy DNA probes were used to screen the libraries and at least two overlapping BAC clones were isolated with each probe from each library, ranging from 45 to 260 kb in insert size. Hybridization of BAC clones with chloroplast DNA probes and fluorescent in situ hybridization using BAC DNA as probes demonstrated that both libraries contain very few clones of chloroplast DNA origin and are likely free of chimeric clones. These data indicate that both BAC libraries should be suitable for map-based cloning of rice genes and physical mapping of the rice genome.

The nucleotide-binding site-leucine-rich repeat (NBS-LRR)-encoding gene family has attracted much research interest because approximately 75% of the plant disease resistance genes that have been cloned to date are from this gene family. We cloned the NBS-LRR-encoding genes from polyploid cotton by a polymerase chain reaction-based approach. A sample of 150 clones was selected from the NBS-LRR gene sequence library and was sequenced, and 61 resistance gene analogs (RGA) were identified. Sequence analysis revealed that RGA are abundant and highly diverged in the cotton genome and could be categorized into 10 distinct subfamilies based on the similarities of their nucleotide sequences. The numbers of members vary many fold among different subfamilies, and gene index analysis showed that each of the subfamilies is at a different stage of RGA family evolution. Genetic mapping of a selection of RGA indicates that the RGA reside on a limited number of the cotton chromosomes, with those from a single subfamily tending to cluster and two of the RGA loci being colocalized with the cotton bacterial blight resistance genes. The distribution of RGA between the two subgenomes A and D of cotton is uneven, with RGA being more abundant in the A subgenome than in the D subgenome. The data provide new insights into the organization and evolution of the NBS-LRR-encoding RGA family in polyploid plants.

Many genes exist in the form of families; however, little is known about their size variation, evolution and biology. Here, we present the size variation and evolution of the nucleotide-binding site (NBS)-encoding gene family and receptor-like kinase (RLK) gene family in Oryza, Glycine and Gossypium. The sizes of both families vary by numeral fold, not only among species, surprisingly, also within a species. The size variations of the gene families are shown to correlate with each other, indicating their interactions, and driven by natural selection, artificial selection and genome size variation, but likely not by polyploidization. The numbers of genes in the families in a polyploid species are similar to those of one of its diploid donors, suggesting that polyploidization plays little roles in the expansion of the gene families and that organisms tend not to maintain their ‘surplus’ genes in the course of evolution. Furthermore, it is found that the size variations of both gene families are associated with organisms’ phylogeny, suggesting their roles in speciation and evolution. Since both selection and speciation act on organism’s morphological, physiological and biological variation, our results indicate that the variation of gene family size provides a source of genetic variation and evolution.

The patatin multicopy gene family encodes the major storage protein in potato tubers and is organized as a single cluster in the potato genome. We sequenced a 154-kb bacterial artificial chromosome (BAC) clone containing a portion of the patatin gene cluster. Two putatively functional patatin genes were found in this BAC. These two genes are embedded within arrays of patatin pseudogenes. Using a chromatin immunoprecipitation method we demonstrate that the dramatic increase of patatin gene expression during the transition from stolons to tubers coincides with an increase of histone H4 lysine acetylation. We used 39 rapid amplification of cDNA ends to profile expression of different patatin genes during tuber development. The profiling results revealed differential expression patterns of specific patatin gene groups throughout six different stages of tuber development. One group of patatin gene transcripts, designated patatin gene group A, was found to be the most abundant group during all stages of tuber development. Other patatin gene groups, with a 48-bp insertion in the 39-untranslated region, are not expressed in stolons but display a gradual increase in expression level following the onset of tuberization. These results demonstrate that the patatin genes exhibit alterations in chromatin state and differential transcriptional regulation during the developmental transition from stolons into tubers, in which there is an increased demand for protein storage.

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