Sex‐Differential Resource Allocation Patterns in the Subdioecious Shrub Hebe Subalpina

Abstract

Vegetative growth and sexual reproduction were compared (under natural conditions and following defoliation) for the two sexual morphs of the subdioecious evergreen shrub Hebe subalpina. Under natural conditions, both morphs produced the same number of flowers, but because the flowers of the polleniferous morph (here called m ales ) are larger males allocated almost twice as much biomass to flower production as did females. Females, by contrast, produced larger fruit as well as more fruit than males. In terms of biomass, females allocated 4.7 times as much as males on fruit production. Overall, females allocated almost twice the biomass to flowers and fruit. The morphs also differed in the timing of their growth, with each morph growing less during the phase in which its reproductive costs were highest. In spite of the greater allocation by females to reproduction, the two morphs grew the same amount over the entire reproductive cycle. This may be a result of the fact that females produced more leaves earlier in the season, which enabled them to accumulate more resources than males. The defoliation experiment substantiated the morph—differential allocation patterns. Defoliated shoots on females fruited, but did not grow relative to controls, and defoliated shoots on males grew, but did not fruit. These differences suggest that divergent selection has been operating on the morphs since the evolution of subdioecy (via gynodioecy) from cosexuality in Hebe, and may be related to differences in the relative fitness gains to the morphs through pollen vs. seeds.