Tropical forest communities are among the most diverse on Earth. A few large genera tend to contribute disproportionately to this diversity. How large numbers of congeneric species coexist in local communities has been a puzzle in community ecology research.

In a study published in Journal of Ecology, researchers from the Xishuangbanna Tropical Botanical Garden (XTBG) of the Chinese Academy of Sciences and their collaborators sought to explore the coexistence mechanism of closely related species. Using spatially explicit soil data and species-level transcriptomic data, they studied 13 Ficus species occurring within a 20-ha forest dynamics plot.

The researchers used detailed data on the functional genetic similarity of the species to assess whether co-occurring congeners are dissimilar in drought and defense-related genes.

Specifically, they sequenced and assembled the leaf transcriptomes of 13 locally co-occurring Ficus species in a tropical rainforest in southwest China and used the data to quantify whether genetic distance between species in genes related to drought and defense were associated with patterns of co-occurrence within and between habitats that differ in their soil water content.

They found widespread and consistent evidence for the importance of defense gene dissimilarity in co-occurring species, providing genetic support for what would be expected under the Janzen-Connell mechanism.

In addition, drought-related gene sequence similarity was associated with Ficus co-occurrence, suggesting that similar responses to drought promote co-occurrence. The differences in defense genes of co-occurring Ficus trees were associated with Ficus co-occurrence, while the similarities in drought-related gene sequences were associated with the co-occurrence of Ficus.

"Our study provides the first detailed functional genomic evidence that, taking Ficus as an example, the co-occurrence of closely related species may be associated with functional gene differentiation in terms of drought and defense. The results also confirm the potential of community transcriptomics in helping to explore the mechanisms underlying biodiversity conservation," said YANG Jie of XTBG.