Researchers at the Indian Institute of Science Education and Research (IISER) Bhopal have carried out whole genome sequencing of banyan (Ficus benghalensis) and peepal (Ficus religiosa) from leaf tissue samples. They also undertook a comprehensive genome-wide phylogenetic analysis with 50 other angiosperm plant species, including four other sequenced Ficus species.
Genome sizes of these two Ficus species were corrected compared to the previously estimated genome sizes. The draft genome assemblies were over 392 Mbp for banyan and nearly 333 Mbp for peepal.
The work helped in identifying 17 genes in the case of banyan and 19 genes of peepal with multiple signs of adaptive evolution (MSA) that play a pivotal role in long-time survival of these two Ficus species. The genes with multiple signs of adaptive evolution came about in response to population bottleneck faced by both trees around 0.8 million years ago. The study has been published in The Journal of Clinical Endocrinology & Metabolism.
Undertaking the comparative evolutionary analyses of closely related plant species helped the researchers in precisely identifying the genes with evolutionary signatures in both plants. Similarly, comparing other plant species with long lifespan in the comparative analysis helped in the identification of adaptively evolved genes, which could have played a significant role in longevity of both banyan and peepal tree species. The inclusion of coding genes from other Ficus species helped the researchers identify plausible species-specific genes responsible for the morphological characteristics and evolutionary adaptation in these two plant species.
“The comparative evolutionary analysis performed across 20 phylogenetically closer Eudicot species revealed adaptive evolution in genes involved in major cellular mechanisms associated with long-time survival such as phytohormones signalling, senescence pathways, fig-wasp coevolution, stress tolerance, which is the highlight disclosure of this study,” says Dr. Vineet K. Sharma, Associate Professor at the Department of Biological Sciences, IISER Bhopal, and the corresponding author of the paper.
Genes showing multiple signs of adaptive evolution in banyan were mainly associated with root development, leaf formation, metabolism, pollen tube and seed development and other developmental processes. The MSA genes of peepal trees were mainly associated with root development, reproduction, metabolism.
“The genes related to root, leaf and reproductive growth that have undergone evolution in these Ficus species explains the well-developed morphological characteristics of these trees,” he says.
Gene family expansion/contraction analysis undertaken by the researchers revealed that the highly expanded gene families of both the species were involved in disease resistance functions in plants. “Disease resistance gene families showed gene expansion as well as high gene expression in these species, which can be used as a signature of long-lived plants,” Dr. Sharma says.
In the case of banyan tree, 15 of 17 MSA genes were also associated with tolerance against environmental stress — drought, oxidative stress, and pathogens. In peepal trees, 17 out of 19 MSA genes were associated with stress tolerance activities.
In addition, the researchers identified seven genes involved in two pathways that produce volatile organic compounds in floral scents which attract wasps for pollination.
“One of the key findings of this study is the identification of signatures of adaptive evolution in genes that are associated with providing longevity in both the species. Particularly, the genes related to sustained growth and development — plant root development, flowering, reproductive growth, and metabolism — showed multiple signs of adaptation,” they write.
The adaptive evolution in genes in two cellular mechanisms might explain the well-developed aerial roots that is unique to banyan trees. These aerial roots help provide much support for the growth and survival of large trees against environmental challenges particularly strong winds that uproot many other similarly large trees.
Both plants show genes with signatures of multiple adaptive evolution involved in phytohormone signalling pathways. These pathways function to regulate plant developmental senescence and ageing processes. This could be one more reason why banyan and peepal trees have a long lifespan. Both banyan and peepal trees have select plant disease resistance gene families that have been expanded through gene duplication events in the course of evolution which confers greater longevity.
Also, 88% and 89% of the MSA genes in banyan and peepal trees, respectively, are associated with tolerance against biotic and abiotic stress responses. This, in turn, helps these plants to survive when faced with environmental challenges.
“To survive in tropical and sub-tropical ecosystems as keystone species, Ficus trees have evolved their developmental and stress tolerance mechanisms,” says Dr. Sharma. “Stress tolerance mechanisms are also responsible for the medicinal properties of these trees.”
“Availability of their genome sequences will aid in further studies on this ecologically important genus and other comparative aspects, including medicinal properties between short-lived and long-lived plants,” says Dr. Sharma.