THE EXPLOITATION OF POLLINATORS BY SPECIES AND HYBRIDS OF PHLOX.

Abstract

Interspecific hybridization is perpetrated by animal pollen vectors in a multitude of plant genera where the floral mechanisms of the participating species can accommodate the same vector. The frequency and implications of hybridization has stimulated investigations on pollinator flower constancy, the basis of ethological and mechanical isolation, and the role of floral isolation in speciation. Many of the problems and contributions of pollination ecology as related to the evolutionary process have been elaborated upon by Grant (1949, 1963), Grant and Grant (1965), Baker (1963), Baker and Hurd (1968), and Faegri and van der Pijl (1966). In spite of the interest in pollination ecology during the past several years, certain aspects of the field remain unattended. When comparing species, or species and their hybrids in terms of their fitness components, little consideration has been given to the mating success of these entities except in relation to the breeding system. Mating success in animal-pollinated outbreeding plants is a function of pollinator service. Foremost among the questions to be answered are the following: Are related species and their hybrids equally efficient in exploiting the pollinator fauna? If there are differences among species and hybrids, to what factors can they be attributed? Answers to these questions were sought from Phlox drummondii Hook. var. drummondii and P. cuspidata Scheele. These annuals are indigenous to central Texas where they are abundant and occasionally in contact. Contact affords an opportunity for hybridization as the species have synchronous flowering periods, similar floral mechanisms, and weak genetic barriers to crossing (Erbe and Turner, 1962). Hybridization has been reported at several localities (Erbe and Turner, 1962; Levin, 1967). Correlative analysis of pollen fertility, chromosome pairing, and flavonoid profiles suggested that the hybrid swarms contaihed little other than the parental species and F1 hybrids (Levin, 1967). The two species are distinctive in reproductive and vegetative morphology, the former offering the most definitive taxonomic characters (Table 1). Their corollas are salverform, similar in conformation and adapted for lepidopteran pollination. They differ in size and pigmentation. Phlox drummondii displays relatively large red corollas; it is the only red-flowered Phlox. In contrast, the coroilas of P. cuspidata are diminutive, being the smallest in the genus, and are pink. Hybrids have corollas which are magenta in color and intermediate to their parents in dimensions. Phlox drummondii is an obligate outbreeder and P. cuspidata a facultative inbreeder (Erbe and Turner, 1962; Levin, unpubl.). The floral morphology of the species and hybrids shows that there are no mechanical barriers to pollen exchange between the entities. The presence of hybrids, and the presence of parental pollen on hybrid stigmas (as will be shown below) attests to the ability of pollinators to effectively transport pollen among the phloxes. It is likely that the phloxes are drawing upon the same pollinator pool. This is not to imply that a pollinator might not prefer one Phlox to another. Unfortunately, observations within hybridizing populations served only to reinforce the observation that P. drummondii is frequently pollinated by Papilio sp. (Whitehouse, 1939). Grant and Grant (1965) have observed the hawk