Evolutionary genetics and conservation of Citrus genetic resources in home gardens in Northeast India

The genus Citrus L. is a major source of commercial fruits, cultivated in tropical and subtropical regions of the world. Many Citrus species and cultivars are commonly found in home gardens and play an important role in supporting the livelihood of local inhabitants in northeast India. This study includes the phylogenetic relationship among Citrus species, population genetics of a medicinally important and native Citrus species (C. medica) and plant diversity in the home gardens in northeast India. The phylogenetic relationships of 24 species of Citrus based on nucleotide sequences of three chloroplasts (trnL-trnF, trnS-trnG and rps16) and one nuclear (ITS2) DNA regions were inferred through three major phylogeny reconstruction methods. The analyses grouped morphologically distinct 24 Citrus species into five phylogenetically defined groups with presence of a true species (C. medica, C. reticulata and C. grandis) and their probable hybrids in three groups. Furthermore, this study revealed two additional groups with two wild, endemic and endangered species (C. indica and C. assamensis). The species of acid and Papeda groups are polyphyletic. The genetic diversity and structure of 219 Citrus medica individuals collected from 8 domestic and 4 wild populations were assessed using 5 polymorphic microsatellite markers. In total 67 alleles were detected with an average of 13.4 alleles per locus. The mean observed and expected heterozygosity values ranged between 0.220 - 0.540 and 0.438 - 0.733 respectively among the wild and domestic populations. Domestic populations showed close genetic relationships as compared to wild populations and pairwise Nei’s genetic distance ranged from 0.062 to 2.091. Analysis of molecular variance (AMOVA) results showed higher genetic diversity among- than within-populations. The analysis of population structure revealed five groups, partly corresponding to geographical location of populations. The admixture of individuals among wild and domestic populations revealed their introgression in populations by natural or farmer mediated agricultural practices. Citrus medica populations in the region are genetically diverse. The eastern Himalayan region of northeast India is well known for its traditional home gardens, which play important role in the maintenance of livelihoods of indigenous communities and conservation of biological diversity. This study determined the plant diversity and their importance in conservation of plant genetic resources. This study was conducted in 90 home gardens located in 6 villages in two different districts in Mizoram and data collected through direct observations and thorough discussions with the farmers. The size of home gardens ranged between 0.10 – 0.60 ha and showed significant (P<0.001) positive correlation between the garden size and plant species diversity. A total of 333 plant species (133 trees, 92 shrubs and 108 herbs) belonging to 122 families with an average of 78 species per home garden were recorded. The species diversity indices for trees, shrubs and herbs were 4.76, 4.39 and 4.58 respectively. The species similarity within each life-form was high with 50% for trees, 38% for shrubs and 49% for herbs. Plant species in the home gardens could be grouped into 11 major use categories and majority of plants were of medicinal or multiple use categories. These home gardens are reservoirs of plant genetic resources and play a vital role in sustaining the livelihood of local inhabitants.