The Research Development of Unique Chromosome in Cowpea (Vigna unguiculata)

The cowpea, Vigna unguiculata (L.) Walp., is an important agronomic crop which constitute a major source of protein for food and is particularly tolerant to drought and heat stress. The cowpea is still poorly understood cytogenetically, so there are many researches which aim to understand the cowpea, especially in its chromosomal structure. 
One of former researches reported that there are polytene chromosomes in the anther tapetal cells of a bean species, including in cowpea¹. The polytene chromosomes in cowpea looked small and displayed a small nucleolus compared to other polytene chromosomes, but they were found in a large number of cells and in a conveniently manipulated tissue.

This unique condition of cowpea’s chromosomes drive many further research in cowpea. In 1996, Guerra reported a developmental research with his friend about rDNA sites in mitotic polytene chromosomes of cowpea revealed by in situ hybridization². They investigated the number of rDNA sites in the mitotic and anther tapetal polytene chromosomes in cowpea by in situ hybridization using a biotinylated  rDNA probe. Then the banding pattern resulted is used to characterize the rDNA bearing chromosomes. Tapetal polytene chromosomes of cowpea were strongly stained with DAPI and showed largely diffuse chromatin without a clear definition of the chromatin thread. There is a large number of rDNA sites observed in cowpea which may have originated by genetic mechanism. In cowpea, four to ten rDNA hybridization sites were visible, though most cells showed six to eight. Variation in the size of the in situ hybridization signals observed in polytene nuclei differed from in mitotic cells. Even in polytene nuclei with the maximum number of ten rDNA sites, no more than six hybridization sites were large, the remaining four were always smaller.

The chromosomal variation in cowpea bring a new research by S. Gnanamurthy and D. Dhanavel (2014). They conducted a research about the effect of ethyl methane sulphonate (EMS) on induced morphological mutant and chromosomal variation in cowpea³. The morphological and chromosomal variation variation was found to be mutagen sensitive in somatic cells of cowpea. It was found to increase as the concentration of EMS increased. The chemical mutagen like EMS induces high frequency of chromosomal changes like anaphasic bridge, anaphasic laggard, anaphasic bridge and clumbing of chromosome were including control plants also observed. The investigation revealed that the isolation of the early maturity with high yield and high protein content is possible in 25 mM of EMS.

The other new research also reported a highly distinct chromosomal structures in cowpea which was revealed by molecular cytogenetic analysis⁴. It is possible trough the development of molecular research during the recent years and also important to develop the cytogenetic research by combining it with the molecular analysis. Aiko and team used molecular cytogenetics to characterize the structure of pachytene chromosomes to advance the knowledge of chromosome and genome organization in cowpea. Their data showed that cowpea mitotic metaphase were small and most were metacentric or sub metacentric. In meiotic pachytene chromosomes derived from cowpea pollen mother cells, there were several interesting cytological features. The centromeres were cytologically larger as compared to other plant species and the centromeric chromatin looked markedly different from heterochromatic and euchromatic regions. Some of the heterochomatin was highly distinct to form “knob-like” structures, particularly around the pericentromeres and telomeres. Pericentromic heterochamatic knob were found flanking all 11 centromeres. Four chromosomal termini had large heterochomatic knob structures and two chromosomal termini possessed small knobs. The chromosomal termini with large heterchomatin have euchromatic subtelomeric regions where homologous chromosomes were not always completely paired and found in early to late pachytene stage. The structure of these chromosomal termini was grouped into three types: unpaired, partially paired, and fully paired. This research also revealed a new comprehensive description of chromosomal structure in cowpea through the molecular cytogenetic analysis.

References

¹ Carvalheira Guerra, Occurrence of Polytene Chromosomes in the Anther Tapetum of Vigna unguiculata (L) Walp., (Journal of Heredity, 1994), 43-46

² Guerra, Kenton, and Bennett, rDNA Sites in Mitotic and Polytene Chromosome of Vigna unguiculata (L) Walp. And Phaseolus coccineus L. Revealed by in situ Hybridization, (Annals of Botany, 1996), 157-161

³ S. Gnanamurthy and D. Dhanavel, Effect of EMS on Induced Morphological Mutants and Chromosomal Variation in Cowpea (Vigna unguiculata (L) Walp), (International Letters of Natural Sciences, 2014), 33-34

⁴ Aiko Iwata-Otsubo, Jer-Young Lin, Navdeep Gill, and Scott A. Jackson, Highly Distinct Chromosomal Structure in Cowpea (Vigna unguiculata), as Revealed by Molecular Cytogenetic Analysis, (Chromosome Res, 2016), 197-216