THE EVOLUTION OF CARBON ALLOCATION TO PLANT SECONDARY METABOLITES - AGENETIC-ANALYSIS OF COST IN DIPLACUS-AURANTIACUS

Diplacus aurantiacus contains large amounts of a leaf phenolic resin, an important deterrent to a leaf-eating caterpillar, Euphydryas chalce dona. The resin can also retard water loss during drought. Furthermore , the leaf resin content differs among plants and populations. This st udy investigates the existence of heritable variation (h(2)) in resin production and tests for a genetic correlation (r(G)) between carbon a llocation to secondary metabolites and growth rate, as well as with th ree other vegetative traits. Nine dam and 10 sire plants were chosen r andomly at a field site and used to generate 78 full-sib families (19 half-sib families) by crossing all males to all females in a factorial design. Heritability was estimated in two ways, and genetic correlati ons were estimated by three methods. We found: (1) the heritability of resin production estimated by the regression of offspring on sires wa s significantly greater than zero (h(s)(2) = 0.32, P < 0.01); (2) the maternal variance in resin content was significantly greater than zero (21.3% of total phenotypic variance); (3) significant negative geneti c correlation between resin content and growth rate was observed from two of three methods and was consistent with the phenotypic correlatio n; and (4) the cost of resin could be assessed quantitatively. The gen etic cost of 1 mg in resin is equivalent to 25 mg of dry shoot-biomass growth, but the phenotypic cost is only 2.1 mg. This study indicates that carbon allocation to these secondary metabolites may respond to n atural selection, and the phenotypic cost of resin production has a ge netic basis in D. aurantiacus. This trade-off suggests that once selec tion occurs, increased phenolic resin production may result in decreas ed growth, or vice versa.