host-associated differentiation

Now that's a mouthful!

I have moved away for looking at local adaptation and my new focus is host-associated differentiation or HAD. What does HAD mean? Although the focus of my reseach has previously been examing at adaptation from a tri-trophic perspective (the plant, the herbivore, the parasitoid), two recent papers have taken a tri-trophic perspective with a famous (well, at least in my world) system (Stireman et al. 2005, 2006). The famous system of which I speak is the golden rod system, Solidaginis altissima and S. gigantean.
Golden rod pictured here:

(from Thomas Schöpke - Institut für Pharmazie)

The stem galling tephritid, Eurosta solidaginis,

(from Tim Craig's Lab)

feeds on golden rod and produce
galls that look like this:

(from Vassar Biology Department)

This is a classic and well-studied system because it presents a case of both phenotypic and genetic host-associated differentiation. There are host-races of tephritids, Eurosta solidaginis, associated with each species of goldenrod (Solidaginis altissima, S. gigantean). So flies on S. altissima have a different genetic profile than flies on S. gigantean. In this study phenotypic differences in adult preference (female fly laying eggs on the plant) and larval performance (maggot eating the plant) were reinforced by genetic differences using both mitochondrial and allozymes (Waring et al. 1990, Brown et al. 1996).

So who cares?

Let's start with what a host race is...
Diehl and Bush provide the most succinct definition of host races;
a population of a species that is partially reproductively isolated from other conspecific populations as a direct consequence of adaptation to a specific host (1984).
If we see differences in different populations on the same species, this may mean that the plant host is driving diversification of this species, i.e. if we give these tephritid flies enough time (in evolutionary terms) then they may become 2 different species. This is a good indicator that plant hosts may be the force driving new species and it may explain why insects that feed on plants are the most diverse organism (Strong et al. 1994), Mitter et al. 1988, Jaenike 1990). This concept is the foundation of my research.

Are plant hosts driving herbivores to diversify and as the herbivores diversify, do the parasitoids also diversify?

I am hoping that if I find host-associated differentiation in a caterpillar, i will also find host-associated differentiation in the parasioid. Host-associated differentiation can be phenotypic or genotypic. This means I could find differences in performance of the caterpillar (i.e. growth rate, pupal mass, # of eggs laid by female) and/or I could find differences in the genetic sequence. Local adaptation examines variation (or differentiation) only at the phenotypic level (more precisely, differentiation at a few loci and not genetically apparent). Looking at both phenotypic and genotypic differentiation will allow me to get a better idea of the extent to which my caterpillars (and then my parasitoids) have specialized on their hosts.

Later this week... my caterpillars and what I am doing during the summer of 2006. Yikes! - which is already 2/3 over.