Abstract
Cowpea (Vigna unguiculata (L.) Walp) is an important legume, particularly in developing countries. However, little is known about its genome or chromosome structure. We used molecular cytogenetics to characterize the structure of pachytene chromosomes to advance our knowledge of chromosome and genome organization of cowpea. Our data showed that cowpea has highly distinct chromosomal structures that are cytologically visible as brightly DAPI-stained heterochromatic regions. Analysis of the repetitive fraction of the cowpea genome present at centromeric and pericentromeric regions confirmed that two retrotransposons are major components of pericentromeric regions and that a 455-bp tandem repeat is found at seven out of 11 centromere pairs in cowpea. These repeats likely evolved after the divergence of cowpea from common bean and form chromosomal structure unique to cowpea. The integration of cowpea genetic and physical chromosome maps reveals potential regions of suppressed recombination due to condensed heterochromatin and a lack of pairing in a few chromosomal termini. This study provides fundamental knowledge on cowpea chromosome structure and molecular cytogenetics tools for further chromosome studies.Electronic supplementary materialThe online version of this article (doi:10.1007/s10577-015-9515-3) contains supplementary material, which is available to authorized users.
Highlights
Cowpea (Vigna unguiculata (L.) Walp) is an important agronomic crop in Africa and other developing countries and is tolerant to drought and heat stress, especially as compared to other legume crops (Hall 2004)
Cowpea was identified as an Borphan crop^ with limited genomic resources (Naylor et al 2004); since a consensus genetic map with high-density singlenucleotide polymorphism (SNP) markers has been developed from 11 mapping populations (Muchero et al 2009; Lucas et al 2011; consensus genetic linkage map version 6 in http://harvest.ucr.edu)
Cowpea chromosome structure pericentromeric regions of all chromosomes were composed of highly condensed heterochromatin blocks with the chromosome arms being mostly euchromatic (Fig. 1b)
Summary
Cowpea (Vigna unguiculata (L.) Walp) is an important agronomic crop in Africa and other developing countries and is tolerant to drought and heat stress, especially as compared to other legume crops (Hall 2004). Cowpea was identified as an Borphan crop^ with limited genomic resources (Naylor et al 2004); since a consensus genetic map with high-density singlenucleotide polymorphism (SNP) markers has been developed from 11 mapping populations (Muchero et al 2009; Lucas et al 2011; consensus genetic linkage map version 6 in http://harvest.ucr.edu). Extensive genomic resources such as bacterial artificial chromosome (BAC) libraries, BAC end sequences (BESs), and a physical map have been constructed. Whole genome sequencing projects on other important Vigna crops such as mung bean (Vigna radiata) and azuki bean (Vigna angularis) have been published (Kang et al 2014, 2015), which will accelerate the development of genetic and genomic resources for these closely related species and allow them to leverage each other to advance our ability to genetically manipulate these crops
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