Ecophysiology and genetics of phenotypic divergence in Solanum

Precipitation is a major axis of climatic niche diversification in tomatoes. I am trying to understand the physiological basis of adaptation to dry and wet habitats.

Precipitation is a major axis of climatic niche diversification in tomatoes. I am trying to understand the physiological basis of adaptation to dry and wet habitats.

More tender leaves (lower leaf mass per area [LMA]) are found in tomato species from drier habitats. Physiological data suggest that this may be an adaptation to increase water-use efficiency by decreasing resistance to CO2 diffusion inside the leaf.

More tender leaves (lower leaf mass per area [LMA]) are found in tomato species from drier habitats. Physiological data suggest that this may be an adaptation to increase water-use efficiency by decreasing resistance to CO2 diffusion inside the leaf.

The physics of CO2 diffusion shape leaf anatomy during adaptation to dry climates

An understanding of physiological mechanisms can often reveal adaptations that would be difficult to detect otherwise. For example, it is commonly assumed that thick/succulent or hard/sclerophyllous leaves are favored in drier climates because they help plants tolerate drought. Among tomatoes, species from the driest habitats actually have the thinnest, most tender leaves (see figure to the right). In collaboration with ecophysiologists, I have shown that tender leaves permit more efficient photosynthesis by improving internal CO2 diffusion without additional H2O loss, increasing water-use efficiency (Muir et al. 2014 & unpub. See figures below). These data implicate selection on leaf anatomy to maximize CO 2 supply as a component of major ecological transitions in this group.

Increasing leaf mass per area (greater density/thickness) decreases internal CO2 diffusion, measured as "mesophyll conductance" or gm.

Increasing leaf mass per area (greater density/thickness) decreases internal CO2 diffusion, measured as "mesophyll conductance" or gm.

We see the same pattern in both tomatoes (coloured points) and across a global dataset.

Genetics of adaptation: two large effect loci drastically alter stomatal ratio

The genetic architecture of trait differences can reveal much about the underlying evolutionary forces. I have studied the genetics of adaptation using functional traits, such as stomatal ratio, which affects photosynthetic efficiency. Importantly, stomatal ratio differs drastically among tomato species, recapitulating most of the variation seen across all land plants and correlates with native precipitation levels. I have found that two large effect loci (Muir et al. in press) transform a hypostomatous leaf (all stomata on the lower leaf surface) into an amphistomatous leaf (stomata on upper and lower surfaces). Both loci were likely fixed by natural selection and contain candidate genes in the stomatal development pathway. The genetic architecture of stomatal ratio closely resembles that of other traits known to underlie major ecological transitions, suggesting a similarly transformative role of this trait in tomato species. Read more about my research on stomatal evolution here.

2## Clade-wide stabilizing selection on biomass allometry supports metabolic scaling theory

Despite wide variation in organism size, allometry between body size and many traits is often conserved, hinting at functional constraints. Using phylogenetic comparative methods, I have shown that the allometric relationship between plant size and leaf area is constrained by stabilizing selection around an optimum not significantly different from that predicted by theory (Muir & Thomas-Huebner 2015).