RESOURCE COMPENSATION AND THE EVOLUTION OF GYNODIOECY IN PHACELIA LINEARIS (HYDROPHYLLACEAE)

Gynodioecy is a dimorphic breeding system in which hermaphrodite and female individuals coexist in populations. Theoretical models have shown that if nuclear genes control sex expression, then gynodioecy can evolve only when females have large advantages in one or more fitness components. These female advantages must be large enough that females' expected lifetime production of viable seeds is more than twice that of hermaphrodites. Previous studies have found that cytoplasmic inheritance and/or a large offspring-vigor advantage of females (caused by hermaphrodite self-pollination and inbreeding depression of selfed seeds) account for this breeding system's evolution. This paper reports studies of gynodioecy in Phacelia linearis, an insect-pollinated annual plant in which gender inheritance appears to be nuclear. Twenty-six P. linearis populations surveyed in northern Utah, USA, contain a majority of perfect-flowered hermaphrodites, but most (22) also contain male-sterile individuals (females), at frequencies of up to 0.16. The hermaphrodite selfing rate is low (0.00-0.20 in four populations). Maternal gender does not consistently affect components of offspring vigor, such as seed size, germination rate, seedling survivorship, and vegetative size. Plants of the two genders do not differ in number of seeds per fruit or mean seed mass. Females produce significantly more fruits and seeds than hermaphrodites in natural populations. The ratio of the mean lifetime seed production of females to the mean lifetime seed production of hermaphrodites ranged from 1.31 to 2.52 in six natural populations. Females have greater shoot biomass than hermaphrodites and produce more seeds at any given shoot biomass than hermaphrodites, suggesting that their seed-production advantage arises from gender-specific patterns of resource allocation to growth and reproduction. The gender difference in plant size varies across environments and across genetic backgrounds. In this species nuclear gynodioecy appears to be evolutionarily stable mainly because of resource compensation by females, without a large outcrossing advantage of females.