Physiological and molecular response to chilling stress in mangroves
Patterns of plant distribution provide a fundamental, unifying framework in which to examine the evolution and ecology of plants, and their influence on global landscapes. Mangrove forest is a tropical/subtropical intertidal plant community that provides important ecosystem services and supports a large portion of the global coastal human population. The distribution of mangroves is mainly restricted by low temperature, thus the warming global climate is expected to expand mangrove distribution range poleward. The distribution range of mangrove species may be shifting at different rates according to their tolerance towards chilling stress.
Mangrove distribution (top) corresponds with sea surface temperature (bottom) of 20 °C isotherm in winter. Adapted from Polidoro et al. (2010) and NASA (2009).
In this project, we aim to:
- Determine the physiological tolerance threshold of various mangrove species towards chilling stress.
- Determine the extent of (molecular) adaptation to chilling stress in populations from varying climatic conditions, especially the amount of adaptive variation that is present in the genome.
- Understand the molecular regulatory network of response to chilling stress in mangroves.
Freeze damage in Kandelia obovata at its northern limits in Kagoshima, Japan, due to extreme winter temperatures in early 2016. The photo was taken in December 2016 by Alison Wee.
Addressing these critical knowledge gaps could elucidate on the biological limitation and evolutionary potential of mangroves growing at the edge of their tolerances, and improve predictive modeling of future mangrove distribution. We use a combination of common garden experiments, plant growth chamber experiments, field physiological measurements, transcriptome analysis and genotyping in this project.
Population genomics and conservation management of threatened species in the Dipterocarpaceae family
Genetic exchanges among individuals and populations, either by pollen or seed, are necessary to maintain the long-term viability of a species. Hence, it is crucial to assess the genetic connectivity of individuals within a population and of populations across landscapes in order to identify priority species / population and to achieve successful conservation. The advent of Next-Generation Sequencing (NGS) technologies offers a powerful approach to quantify gene flow across landscapes in non-model species, making it increasingly feasible to investigate the genetic variation of threatened species, its relationship to ecological and landscape conditions, and to evaluate management strategies.
Diperocarpaceae is a mega-diverse, pan-tropical plant family distributed in tropical and subtropical Asia, Africa, and South America, with Asia being the present centre of diversity. Asian dipterocarps are one of the tallest tropical trees. They dominate the forest canopy, have straight stems and are highly valued for their hardwood timber. As a result, dipterocarps in lowland forests have been heavily logged, leading to extensive habitat destruction and fragmentation. Over-harvesting severely jeopardizes the ecological roles of dipterocarps as the structural foundation and keystone species of tropical forests, and as carbon sinks. Many dipterocarp species are threatened due to the loss of habitat: circa 58% of all dipterocarps have been assessed by the IUCN Red List, of which 94% are listed as endangered.
Genetic exchanges among individuals and populations, either by pollen or seed, are necessary to maintain the long-term viability of a species. Hence, it is crucial to assess the genetic connectivity of individuals within a population and of populations across landscapes in order to identify priority species / population and to achieve successful conservation.
In this project, we aim to
1.Determine the extent of genetic erosion across natural populations.
2.Determine the impact of fragmentation on gene flow.
3.Determine the role of chilling stress as a selective pressure driving population divergence.
Ultimately, this project presents a targeted research on essential genetic processes of conservation concern: genetic erosion, gene flow and adaptation. This strategy could potentially be integrated into the conservation and restoration of not only dipterocarp species, but also other endangered tropical trees.
Global phylogeography of mangroves
Mangroves are a tropical and subtropical intertidal plant community that could serve as an ideal study system to examine the drivers of landscape-level genetic connectivity. First, mangroves consist of phylogenetically-unrelated plant groups that underwent convergent evolution to adapt to an intertidal environment. Therefore, a wide spectrum of ecology and dispersal characteristics exists across species. Second, being coastal plants, mangroves have undergone dramatic species range changes in the past due to the fluctuations in sea level, especially during the Pleistocene. This gave rise to pronounce phylogeographic patterns and allowed us to examine both the past and present effect of vicariance and landscape features (e.g. land mass). Third, all true mangroves have water-dispersed propagules (i.e. seeds, fruits or viviparous seedlings). Therefore, any disparity in phylogeographic pattern is most likely due to ecological and biological factors.
In this project, we aim to
- Determine the global phylogeographic patterns of multiple mangrove species
- Examine the effect of major of drivers of gene flow via a comparative phylogeographic approach
Putative genetic (left), admixture maps (right) and location of barriers to gene flow (red lines) of 25 populations of Sonneratia alba from the Indo-West Pacific region at three different clustering scenarios (Adapted from Wee et al. 018 Forests). The global phylogeographic pattern of S. alba revealed the effect of vicariance and oceanic barriers in shaping the present-day genetic structure of this species.