Abstract
Analysis of karyotype, nuclear DNA content and RAPD markers were performed in four species of Bruguiera (Rhizophoraceae) of Bhitarkanika mangrove forests, Orissa, India. Detailed karyotype analysis revealing 2n=34 in B. cylindrica and 2n=36 in B. gymnorrhiza was reported for the first time and 2n=34 in B. parviflora and B. sexangula was confirmed. On the basis of the common types of chromosomes present among Bruguiera, two distinct groups were found; one consists of B. cylindrica and B. parviflora and the other of B. gymnorrhiza and B. sexangula. The symmetrical karyotype with same chromosome types grouped B. cylindrica and B. parviflora together and presence of Type E chromosomes placed B. gymnorrhiza and B. sexangula in a separate group, suggesting their closer affinity in their respective group. Analysis of chromosome length, volume, INV and 4C DNA content confirmed this division. Nuclear DNA content was two-fold higher (approximately 17.0 pg) in the second group than in the first (approximately 8.0 pg). The amplification products generated through RAPD revealed 1-9 amplicons with size variations from 600 bp to 2 500 bp with 49.31% genetic similarity between B. gymnorrhiza and B. sexangula and 47.10% in between B. cylindrica and B. parviflora. The high copy number marker band (approximately 1,100 bp) yielded in OPN-15 primer in B. parviflora the characteristic DNA marker, which was cloned and used as probes for assessment of genetic diversity, and demonstrated its close genetic affinity to B. cylindrica. B. gymnorrhiza and B. sexangula also produced similar marker bands of approximately 600 bp and approximately 2,200 bp in the same primer. All of the cytological, 4C DNA content and RAPD data confirmed the existence of two taxonomically distinct groups of Bruguiera: one consisting of B. cylindrica and B. parviflora and the other of B. gymnorrhiza and B. sexangula as placed earlier (1862) in the tribe Rhizophoreae by Bentham and Hooker, on the basis of the flowering habits ofBruguiera. Genetically, the B. sexangula and B. gymnorrhiza group was found to be very closely, rather than distantly, related to B. parviflora and B. cylindrica. Our results demonstrate that molecular markers together with cytological evidence provide an effective tool to access the existing interspecific genetic polymorphism in mangrove species, to solve the taxonomic problems and to design their conservation strategy.
Highlights
Mangroves are perennial plants found in the intertidal zones of coastal wetlands in tropical regions; many species that are found exclusively there are called true mangroves and are capable of forming dense pure stands of trees
The present study deals with detailed karyotype analysis, estimation of 4C DNA content and RAPD polymorphism analysis in the four species of Bruguiera belonging to two morphological groups of the tribe Rhizophoreae in order to ascertain their genetic diversity and to establish phylogenetic relationships as well as affinities, if any, of the four species of Bruguiera belonging to the two taxonomic groups and to resolve the taxonomic status of these genera using the existing taxonomic classification
Karyotype analysis: The somatic chromosome number varied from 2 n = 34 in B. cylindrica, B. parviflora and B. sexangula to 2 n = 36 in B. gymnorrhiza
Summary
Mangroves are perennial plants found in the intertidal zones of coastal wetlands in tropical regions; many species that are found exclusively there are called true mangroves and are capable of forming dense pure stands of trees. The genus Bruguiera, a true mangrove, of the family Rhizophoraceae consists of four species distributed throughout the coastal regions of India: B. cylindrica, B. gymnorrhiza, B. parviflora and B. sexangula. The first species sometimes has solitary flowers, the second sometimes has flowers in pairs and the size difference between flowers is slight; there is a fairly sharp distinction between the species on the basis of flower orientation in relation to different types of pollinators (birds vs insects). Despite this variation, there is not sufficient justification to revise the classification. The present study deals with detailed karyotype analysis, estimation of 4C DNA content and RAPD polymorphism analysis in the four species of Bruguiera belonging to two morphological groups of the tribe Rhizophoreae in order to ascertain their genetic diversity and to establish phylogenetic relationships as well as affinities, if any, of the four species of Bruguiera belonging to the two taxonomic groups and to resolve the taxonomic status of these genera using the existing taxonomic classification
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