VARIATION IN SEX EXPRESSION IN CANADA YEW (TAXUS CANADENSIS)

Abstract

Sex expression was measured in several Canada yew (Taxus canadensis Marsh.) populations of the Apostle Islands of Wisconsin and southeastern Minnesota to determine the extent of variation within and among populations. Sex expression was recorded qualitatively (monoecious, male, or female) and quantitatively (by male to female strobilus ratios or standardized phenotypic gender). No discernible trends in differences in sex expression among populations or habitats were recorded. Trends in sex expression of individuals within populations were complex. Small yews tended to be male or, if monoecious, had female‐biased strobilus ratios. Large yews were monoecious but had male‐biased strobilus ratios. Phenotypic gender, recorded as relative maleness, however, was negatively, but weakly, correlated with plant size. Gender distribution in four of five populations was bimodal, suggesting that cosexual populations consist of male and female morphs. Strobilus ratios of individuals in Apostle Island populations showed significant annual variation, but gender for these same plants was significantly correlated from year to year. Annual adjustments in gender were most pronounced in small yews. The results indicate that relative investment in male and female reproductive structures by Canada yew individuals is responsive to environmental variation, but sex expression also has a proximate genetic component.