Abstract
Asparagus bean (Vigna. unguiculata ssp. sesquipedialis) is a distinctive subspecies of cowpea [Vigna. unguiculata (L.) Walp.] that apparently originated in East Asia and is characterized by extremely long and thin pods and an aggressive climbing growth habit. The crop is widely cultivated throughout Asia for the production of immature pods known as ‘long beans’ or ‘asparagus beans’. While the genome of cowpea ssp. unguiculata has been characterized recently by high-density genetic mapping and partial sequencing, little is known about the genome of asparagus bean. We report here the first genetic map of asparagus bean based on SNP and SSR markers. The current map consists of 375 loci mapped onto 11 linkage groups (LGs), with 191 loci detected by SNP markers and 184 loci by SSR markers. The overall map length is 745 cM, with an average marker distance of 1.98 cM. There are four high marker-density blocks distributed on three LGs and three regions of segregation distortion (SDRs) identified on two other LGs, two of which co-locate in chromosomal regions syntenic to SDRs in soybean. Synteny between asparagus bean and the model legume Lotus. japonica was also established. This work provides the basis for mapping and functional analysis of genes/QTLs of particular interest in asparagus bean, as well as for comparative genomics study of cowpea at the subspecies level.
Highlights
There are two main cultivated divisions of cowpea worldwide, i.e. the dominant subspecies unguiculata, which is widely cultivated in Africa, India, Middle East and the Americas for dry grain and which is a major source of dietary protein for millions of people in developing countries [2], and the subspecies sesquipedialis, which is known as asparagus bean or ‘yard long’ bean in eastern and southern Asia, for production of immature green pods [3]
Asparagus bean is characterized by its striking long (0.5–1 m) pods that are harvested at an immature ‘snap’ stage and its climbing growth habit that is rare in ssp. unguiculata, and is considered one of the top ten Asian vegetables
Diverse landrace forms of asparagus bean are found throughout Asia [4], and yet this subspecies is never observed in Africa, suggesting this form of cowpeas arose in Asia from an unknown progenitor form of cowpea long ago
Summary
Unguiculata, the main objective in asparagus bean breeding is to obtain more pods per plant and to slow the rate of seed development to prolong the production of attractive immature pods favourable for vegetable use. Unguiculata was constructed based on 928 EST-derived SNP markers, providing a solid basis for gene/QTL mapping and genome characterization [6]. The cowpea consensus map alone may not be sufficient to investigate traits of particular interest in asparagus bean given that alleles conferring very long pods and climbing habit likely are absent in the ssp. A comparable genetic map of asparagus bean is needed to identify genes/QTLs related to specific traits relevant to long bean improvement, as well as to reveal the extent of genome conservation/differentiation between the two subspecies. We report the construction of a high-density genetic map for asparagus bean employing a combination of the SSR markers and EST-derived SNP markers
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