Sex Ratios and Coning Frequency of the Cycad Zamia pumila L. (Zamiaceae) in the Dominican Republic

Abstract

In January 1985, features of coning behavior were examined in three populations of the cycad Zamia pumila (Zamiaceae) growing in different environmental conditions in the Dominican Republic. One population exists in a state of semi-cultivation in the relatively open lawns of the National Botanical Garden, Santo Domingo. A second occurs in moderately open areas at the edge of a second-growth forest at Boca Chica, and a third occupies shaded areas under the canopy of a relatively undisturbed forest at Bayahibe. One-third of the botanical garden females bore cones from the current and previous season, whereas only 9 percent of the Boca Chica females showed successive coning and none of the Bayahibe plants did. Four percent of the mature individuals in the botanical garden showed no evidence of coning in either season, whereas one-third and two-thirds of those in the latter two populations, respectively, fell into this non-coning category. The phenotypic sex ratio for the botanical garden and the Boca Chica populations was 1:1. Since nearly all individuals at the first site produced cones and thus were assignable to sex, the genotypic sex ratio of that population was 1:1. A male bias was evident at Bayahibe, where 73 percent of the coning individuals were male. Where environmental conditions are most favorable, the energetically costly female cones appear to be produced more frequently and by a greater proportion of females than where these conditions are less favorable. These environmental differences thus influence the phenotypic sex ratio, which is equality under good to excellent environmental conditions, but male-biased under less favorable environmental conditions under which fewer females than males produce cones each season. DESPITE CONSIDERABLE RECENT INTEREST in the population biology of dioecious seed plants, little attention has been paid to gymnosperms, even though some of these, such as the cycads, show striking sexual dimorphism in their allocation of resources to cones during episodes of sexual reproduction. Of the 75 taxa of seed plants for which published sex ratios were summarized by Willson (1983), only two are gymnosperms (two species of Dacrydium, Podocarpaceae). Since Willson's summary appeared, a study including data on sex ratios and coning frequencies for a population of the New World cycad Zamia pumila L. in Puerto Rico was published by Newell (1983), and another has been published dealing with similar features of the cycad Macrozamia riedlei (Fisch. ex Gaud.) C. A. Gardner in Western Australia (Ornduff 1986). The present paper is concerned with coning behavior and sex ratios in Zamia pumila, and extends observations from the Puerto Rican population studied by Newell to three populations growing under different environmental conditions in the Dominican Republic.