Molecular Cytogenetic Characterization of Cowpea (Vigna unguiculata (L.) Walp) Genome

Cowpea (Vigna unguiculata (L.) Walp) is an important legume with a relatively small genome size of 620 Mbp, consisting of 2n=22 chromosomes. Despite its importance, little is known about its genome and chromosome structure. In this study, we characterized cowpea pachytene chromosomes to advance our knowledge of chromosome and genome organization of cowpea. Microscopic observation revealed several interesting and unusual structures of meiotic pachytene chromosomes including knob-like heterochromatin at pericentromeres and chromosomal termini, unpaired chromosomal termini, and relatively large primary constrictions (centromeres). We integrated the cowpea linkage groups with pachytene chromosomes by physically mapping genetically-anchored bacterial artificial chromosomes (BACs) using fluorescence in situ hybridization (FISH). We characterized cytological features of individual chromosomes, and revealed discrepancies of the BAC positions between on the linkage maps and on pachytene chromosomes. Furthermore, we analyzed the repetitive fraction of cowpea genome present at pericentromeric regions, confirming that a LTR retrotransposon is a major component of pericentromeric regions of cowpea. Immunostaining of 5-methylcytocine showed pericentromeric heterochromatic regions are highly methylated. This study provides fundamental knowledge of cowpea chromosome structure and molecular cytogenetics tools for further chromosome studies. In addition, this study will assist comprehensive understanding of cowpea genome and chromosome structure combined with an ongoing genome sequencing project.