Within-Population, Within-Host Species, and Within-Host Tree Sex Ratios in Mistletoe (Phoradendron tomentosum) in Central Texas

Abstract

Phoradendron tomentosum is a dioecious, evergreen, hemiparasitic plant that infests several host species in central Texas. Sex ratios were investigated in 14 populations on six host species. A female-biased sex ratio of .360 (males/total, N = 1624) was found over all populations. Significant female-biased ratios were also found for mistletoe plants on the major host species and in five of the 14 populations. The relationship between within-tree sex ratio and number of mistletoes in an individual tree indicates greater female bias in trees with heaviest mistletoe infestations. Differential mortality, cytological mechanisms and patterns of dispersal are discussed as possible explanations of the observed female-biased sex ratios. INTRODUCTION Although males and females are expected to occur in equal numbers in dioecious flowering plants (Fisher, 1930), female-biased sex ratios have been reported in several species (Godley, 1964; Mulcahy, 1967; Lloyd, 1974). In plants, deviations from a 1:1 adult sex ratio have been explained as the result of unusual genetic mechanisms (chromosomal or genic; e.g, Barlow and Wiens, 1976), differential pollen tube growth (certation; Smith, 1963; Rychlewski and Zarzycki, 1975; Correns, 1928), differential competitive ability of male and female fertilized ovules (Conn and Blum, 1981), microhabitat differences (Freeman et al., 1981; Grant and Mitton, 1979), differential mortality (reviewed by Lloyd and Webb, 1977; Melampy and Howe, 1977; Onyekwelu and Harper, 1979) or variation in flowering schedules reflecting an apparent, but not true, bias (Meagher, 1981; Meagher and Antonovics, 1982). Phoradendron tomentosum (D. C.) Gray (Viscaceae) is a dioecious, evergreen, hemiparasitic plant, commonly found on hackberry (Celtis L.), ash (Fraxinus L.), sycamore (Platanus L.), cottonwood (Populus L.) and mesquite (Prosopis L.). The species occurs from Oklahoma S into Mexico, E to Louisiana and W to W Texas. It is the only mistletoe species reported within the study area (Correll and Johnston, 1970). This paper reports within-population, within-host species and within-host tree sex ratios of Phoradendron tomentosum in central Texas. The various explanations listed above are considered as potentially influencing the observed ratios. Since mistletoe occurs in distinct populations on host trees, an unusual situation is presented for analyzing possible causes of a deviant sex ratio. MATERIALS AND. METHODS Fourteen populations of mistletoe were sampled near Austin, Texas, between December 1981 and February 1982. Host populations included more or less pure stands of Prosopis glandulosa Torr. and mixed stands of Celtis laevigata Willd., C. reticulata Torr. and Ulmus crassifolia Nutt. Occasional occurrences of mistletoe on Maclura pomifera (Raf.) Schneid. and Sapindus saponaria L. were included in the counts in some populations. Due to the sporadic occurrence of mistletoe in most of the host tree populations, no transect or randomized point method of sampling would have been efficient. Therefore, populations were exhaustively sampled by walking in a criss-cross pattern through the host population, and sexing all mistletoe plants encountered in infected trees. The 'Present address: Biological Sciences Group, U-43, University of Connecticut, Storrs, Connecticut 06268.