Background/Question/Methods
Mangroves, world wide, form a unique ecosystem of intertidal forests which serve a multitude of ecological functions. Mangrove species have evolved independently from non-mangrove plant lineages beginning 69 Ma. Convergent evolution has led these species to a common habitat, representative of extreme environmental conditions. The mangrove community shows diverse physiological tolerance strategies to thrive in these extreme environments. However, mangrove genomes are virtually unexplored, and therefore, present a great resource in the search of novel stress tolerant mechanisms. We have selected two diverse mangrove species to investigate both common and unique transcriptome adaptations to the mangrove habitat: Rhizophora mangle (Rhizophoraceae) is the keystone plant and the most dominant species in neotropical coastal ecosystems; and Heritiera littoralis (Malvaceae) is a eudicot in the old world tropics that displays a unique mechanism for extreme salt exclusion from its leaves. RNA of both species was extracted from different tissue types, growth conditions, and various developmental stages. Purified high quality mRNA was then made into two normalized libraries and sequenced with Roche 454 high-throughput pyrosequencing. Additional ten 384-well plates were sequenced with Sanger capillary sequencing for further coverage and quality checks of the cDNA libraries.
Results/Conclusions
454 sequencing results show ~230,000 sequences for each species with the average read length of 200bp. The results are being analyzed, annotated, and assembled into contigs. A randomly selected sample of 1000 sequences show 5% and 10% redundancy in R. mangle and H. littoralis respectively. Preliminary results show that the majority of R. mangle and H. littoralis sequences share high homology with more studied tree species genomes such as Populus and Gossypium, respectively. Interestingly, the mangrove sequences also share greater homology with less explored plants and especially tree species adapted to extreme environments. However, 75% of mangrove gene models have significant homologs in the Arabidopsis genome which facilitates further annotation. Preliminary analysis of the Gene Ontology (GO) functional categories show that transcripts for response to abiotic or biotic stimulus are 9% in mangroves compared to 3.7% of Arabidopsis. A similar pattern is observed for transcripts indicating stress response, where mangroves show 7% compared to 2.7% in Arabidopsis. Significant differences are also observed in metabolite and nutrient transport between mangroves and Arabidopsis. All mangrove sequences are currently being functionally categorized and further analyzed with a special focus on stress related genes and pathways that would exemplify the mangrove lifestyle.
