Michael Foisy and colleagues contributed a paper to our latest special feature: Macroevolutionary perspectives on biotic interactions. Find out more about their paper below.
A brief history of the project
When an insect chomps on a plant, sometimes the plant tissue will release latex or resin – a “goo” that can be toxic, difficult to digest, and a real nuisance for insect mouthparts. Latex and resin are stored in pressurized canals, which, when damaged, rupture and spill their contents out in dramatic fashion (Figure 1). These canal systems have evolved many times across plants, and play an important role in defence against herbivorous insects.
In 1991, Farrell et al. conducted a now classic study testing whether plants with these pressurized canals are more diverse than closely related plants without canals. Their results were striking: 13 of the 16 comparisons revealed higher species richness when canals were present, supporting the prediction that plant defensive traits are associated with higher diversification rates (i.e. the accumulation of species over time, estimated as the net of speciation minus extinction) in plants.
Their study was one of the first to show strong support for a classic hypothesis about how coevolution between plants and insects could generate biological diversity (Ehrlich & Raven, 1964), and their work remains a prominently cited empirical example even today.
Retesting Farrell et al. 1991
In the three decades since the original study by Farrell et al. (1991), major changes to plant systematic relationships have occurred, and we now have phylogenetic comparative methods that go beyond the sign test they used. In particular, we can explicitly model trait evolution and/or diversification rates. Therefore, our goal was to reassess Farrell’s classic test of whether these defensive “goos” are associated with increased species diversity, but to do so using modern methods and data.
We collected data on the occurrence of latex and resin canals in approximately 300 families and 1000 genera of plants, and used these data (along with updated phylogenetic hypotheses for plant relationships) to test the association between canal evolution and lineage diversification rates. We tested this hypothesis at multiple scales: (i) between clades and (ii) within clades.
As usual, biology is complicated
When we updated and added to the Farrell dataset, interestingly, we no longer found strong support for a relationship between canal evolution and lineage diversification. This was surprising, and so we wanted to zoom in on some of these comparisons, to look at the tempo of diversification in relation to trait evolution.
Within clades, we had enough data to look at two groups: Papaveraceae (poppies) and Araceae (arums). In poppies, the timing of latex evolution coincided with a burst of diversification; however, in Araceae, canal evolution appears to be entirely out of tempo with diversification.
Figure 2 is a simplified version of the figure from our paper. Canal origins are indicated by stars (yellow = latex, green = resin), while shifts in diversification rate are indicated by circles (red = increase, blue = decrease). The relative rates of diversification are shaded in grayscale (darker = higher rates). These data show that diversification coincides with latex evolution in poppies (left), but is uncoordinated with latex and resin evolution in arums (right).
Thus, it seems that latex and resin canals may not be as consistently replicable drivers of lineage diversification across plants as we previously thought. Future studies could follow up on this work as more data becomes available, for example using more sophisticated models of trait dependent diversification, or taking other factors (such as clade age) into account. We are looking forward to the day that more data are available, so that this association can be even more rigorously examined in even more clades. But in the meantime, latex and resin remain super cool defensive traits with large consequences for plant fitness. Who doesn’t want a powerful goo weapon to fight their enemies?!
Michael Foisy, Loren Albert, Daniel Hughes & Marjorie Weber.