Biotechnology of stylosanthes

Abstract

The genus Stylosanthes contains some 40 species largely classified on morphological characteristics and has a monophyletic origin, closely related to Arachis with the S. guianensis species complex as the most ancient group clearly distinct from the rest of the genus. Many species are important forage legumes and grown both in annual and perennial system of agriculture. Species are grouped in two subgeneric sections, sections Styposanthes and Stylosanthes based on the presence or absence, respectively, of an axis rudiment, a small appendage at the base of the pod or loment. Most species of Stylosanthes are diploid (2n = 20) but polyploid species (2n = 40 and 2n = 60) also exist. Latter are exclusively allopolyploid. Section Styposanthes contains both diploid and polyploid species, while species in Section Stylosanthes are exclusively diploid. Molecular markers have proved that a tertraploid (4n) is a combination of a diploid (2n) species from section Stylosanthes and a diploid (2n) species from Sec. Styposanthes. A reasonable numbers of sequence-tagged site (STS) markers have been developed in this genus and used to define the progenitors of tetraploid S. scabra, S. hamata and only hexaploid species S. erecta. Genetic diversity study in this genus is largely based on isozyme, RAPD and STS markers. Lack of sufficient EST sequences and simple sequence repeat (SSR) markers have hampered a desired good linkage map; however, RAPD and few RFLPs based map delivered QTLs for drought and anthracnose disease. Before the introduction of Stylosanthes seabrana in year 1998-1999, S. scabra, S. hamata, S. guianensis and S. humilis were major species cultivated in Indian tropics and sub-tropics. Though regeneration protocols have been developed for few species, gene transfers through genetic engineering means have been very limited in this genus. Incorporation of leaf meal, stylo meal (as the replacements of concentrate) in feeding system of small and large ruminants, poultry ration as well as their conservation in form of fodder banks are some of the new and impacted initiatives have been taken with Stylosanthes crop in South Asian countries. Narrow genetic base, insufficient SSR markers, loose genetic map and overall low breeding efforts world wide in Stylosanthes invite concerted attention for its improvement by exploiting genomic resources and information being generated with related legumes, vital to this less explored but an important forage legume for better and improved livestock husbandry.