Reproductive Ecology of Opuntia Macrocentra (Cactaceae) in the Northern Chihuahuan Desert
Mandujano, Maria C., Golubov, Jordan, Huenneke, Laura, The American Midland Naturalist
We studied the floral biology, floral visitors, levels of florivory, and mating system of Opuntia macrocentra in a population of ca. 300 individuals in order to describe what factors affect flower/ fruit ratios. Blooming for the species occurred once a year during spring. Flowers were hermaphroditic, produced nectar, and remained open 6 to 9 h during a single day. Anther dehiscence starts at flower aperture and stigma receptivity starts approximately 1 h later. The most important floral visitors were solitary bees from the Anthophoridae family (genus Diadasia). Open- pollinated control and cross pollination treatments had the highest fruit set (96.8 ± 3.2% and 83.9 ± 6.7%, respectively), but fruit set for forced self-pollination treatment (77.4 ± 7.6%) did not differ from the cross-pollination treatment. Seed production was also highest in the open-pollinated treatment; the average number of seeds per fruit in the open-pollinated treatment was 40% higher than the cross-pollinated treatment and 64% higher than the self-pollinated treatment. The flowers were self-compatible and did not require a visitor to set fruit. Flower/fruit ratio was slightly above one over all pollination treatments (fruit ratios between 1.0-1.3), suggesting that almost all flowers turned into fruits. Outcrossing rates suggest a mixed mating system, but inbreeding depression was found for both fruit and seed set. Developing fruits were consumed by the caterpillar (Lepidoptera: Olycella subumbrella) and decreased fruit set from 20% to 100%. Florivory and inbreeding depression were the major factors that decrease fruit set for this species.
Flower and fruit production are two closely related processes that determine reproductive success in angiosperms, but the factors as well as sources of variation that determine either flower or fruit production can be very different across species. Angiosperms consistently produce many more flowers than fruits and understanding what factors control fruit/ flower ratios in plants and the adaptive significance of such ratios has been an active area of research since the 1980s (e.g,. Lloyd, 1980; Lloyd et al., 1980; Stephenson, 1981; Sutherland and Delph, 1984; Sutherland, 1986a, b; Agren, 1988; Campbell, 1989; Guitian, 1994; Martínez-Peralta and Mandujano 201 1 ) . Fruit set depends on environmental conditions and resources (i.e., precipitation, temperature) (Trueman and Wallace, 1999; Bowers, 1996; Pina et al., 2007) and biotic factors such as pollinator availability (Rathcke, 2001), pollinator behavior, and specificity (Marr et al., 2000; SchiesÜ and Schlüter, 2009) plant size (Bowers, 1996; Mandujano et ai, 2001) florivory, and mating system (Holsinger, 1992; Bowers, 1997; McCall, 2008; Pina et ai, 2010). The latter will largely depend on genetic factors such as selfincompatibility (Elam et ai, 2007) and inbreeding depression (Bellusci et ai, 2009; Strong and Williamson, 2007).
Flowering plants exhibit a variety of sexual systems (Darwin, 1859; Lloyd, 1979; Schemske and Lande, 1985; Holsinger, 1992; Richards, 1997; Barrett, 2003). Reproduction through seeds vs. clonal reproduction can affect opportunities and selection pressure for selfing vs. outcrossing (Bowers, 1996). Compared to nonclonal species, plant species with clonal reproduction can have more opportunities for selfing due to spatial clustering of clones (Carillo-Angeles et al., 2011). Developing a more complete database on mating systems of clonal plant species would help address whether selfing rates or selection pressures against selfing (such as inbreeding depression) are actually lower in clonal species. Even though clonality is widespread in vascular plants and found in several genera in the Cactaceae (e.g., Backebergia, Cylindmpuntia, Echinocereus, Lophophora, MammiUaria, Opuntia, Stenocereus) , the importance and effect on the breeding and mating systems has only recently been explored (Carrillo-Angeles et al. …