Merging ecology and phylogeny: soil endemism, soil chemistry, and plant chemical defense in Californian mustards (Streptanthus s.l.)

Background/Question/Methods

Integration of phylogenies with clade-wide ecological data collections is expanding our ability to address classic ecological questions.

A main paradigm to explain soil endemism is that soil specialists are excluded from benign soil environments by tradeoffs in competitive ability arising from physiological adaptations to soils (competitive-exclusion hypothesis). It has also been suggested that harsh substrates might be a refuge from pathogens (pathogen-refuge hypothesis), that enemies might mediate soil specialization because tissue is costly to replace in harsh environments (defense hypothesis), and that apparency might select for increased defense in such environments (defense-apparency hypothesis).

For a clade of ca. 60 Californian mustards (Streptanthuss.l.) of which up to 1/3 are endemic to serpentine soils, we have inferred evolutionary relationships and history of serpentine use (endemism has evolved 4-5 times; tolerance, 9-11 times).

We analyze glucosinolate profiles– chemical plant defenses– in the context of our phylogeny, soil use, and microhabitat characteristics of each species. We ask: 1.Which elements of chemical defense are more evolutionarily conserved?; 2.Are serpentine use or microhabitat harshness (bareness/rockiness/chemical soil composition) predictive of plant defense?; 3.Is there evidence of a tradeoff between serpentine use or habitat harshness and defense such that plants from these environments are more chemically defended?

Results/Conclusions

Glucosinolate structure and amount are highly variable among species. The three main classes of glucosinolates – indolic, aromatic, and aliphatic– are represented in species profiles of Streptanthus s.l., with the aliphatic glucosinolates being clearly predominant. The degree to which the glucosinolates’ side chain is elongated and oxidized is variable as well, as in other members of Brassicaceae.

Integrating over phylogeny, our data on plant chemical defense suggests that the relationships between substrate use, habitat quality and plant chemical defense are complex. There is high evolutionary lability in glucosinolate profiles (values of phylogenetic conservatism tend to be low). This trend includes diversity of compounds, total amount of glucosinolates produced, and individual compounds in the profiles of analyzed species.

To date, we have found a trend for aliphatic glucosinolates to be related to serpentine substrate, but no strong relationships between glucosinolate content and microhabitat aspects of bareness or serpentine soil; additional analyses are ongoing relating glucosinolates to soil chemistries and other aspects of soil endemism.