Construction of BAC libraries from two apomictic grasses to study the microcolinearity of their apospory-specific genomic regions.

We have constructed bacterial artificial chromosome (BAC) libraries from two grass species that reproduce by apospory, a form of gametophytic apomixis. The library of an apomictic polyhaploid genotype (line MS228-20, with a 2C genome size of approximately 4,500 Mbp) derived from a cross between the obligate apomict, Pennisetum squamulatum, and pearl millet ( P. glaucum) comprises 118,272 clones with an average insert size of 82 kb. The library of buffelgrass ( Cenchrus ciliaris, apomictic line B-12-9, with a 2C genome size of approximately 3,000 Mbp) contains 68,736 clones with an average insert size of 109 kb. Based on the genome sizes of these two lines and correcting for the number for false-positive and organellar clones, library coverages were found to be 3.7 and 4.8 haploid genome equivalents for MS 228-20 and B12-9, respectively. Both libraries were screened by hybridization with six SCARs (sequence-characterized amplified regions), whose tight linkage in a single apospory-specific genomic region had been previously demonstrated in both species. Analysis of these BAC clones indicated that some of the SCAR markers are actually amplifying duplicated regions linked in coupling in both genomes and that restriction enzyme mapping will be necessary to sort out the duplications.