Reproductive Biology of Grusonia bradtiana (Cactaceae): A Dominant Species and Endemic Clonal Cactus from Cuatro Ciénegas Basin and Contiguous Areas in the Chihuahuan Desert

Grusonia bradtiana “viejito” (old man cactus) is an endemic species from Cuatro Cienegas Basin and nearby areas. Grusonia includes 17 clonal species distributed along North American deserts which grow in dense cushion or shrubs. Grusonia bradtiana reproduces sexually by flowering and fruiting, forming seeds with new genetic combinations (new genets). The species clones by fragmentation of stems of different sizes that root independently, producing genetically identical offspring (ramets). Clonal species develop complex reproductive interactions as pollination output depends on pollen transfer between genetically different genets or identical ramets. The hypothesis is that clonality negatively affects sexual reproduction as floral traits are adaptations to promote cross-pollination (among genets) and have evolved to reduce negative effects of inbreeding. We studied the reproductive biology of Grusonia bradtiana and assessed the effect of clonality upon its reproductive success with controlled pollination. We also determined the frequency and taxonomic identity of floral visitors, to assess the pollination syndrome. Flowering occurs once a year during spring. Flowers are diurnal with a life span of 8 h; they are yellow, with radial symmetry, yellow-white lobulated stigma, and produce nectar. Flowers have thigmonastic stamens with red filaments supporting anthers that contain high amount of viable pollen. The flower is perfect, there is no separation of sexual functions in time (dichogamy), but there is in space (herkogamy), attributes that allow selfing and may reduce sexual interference, respectively. The fruit is dry, possibly a trait unique to Grusonia. Pollinators are solitary bees (Diadasia and Melissodes), a melittophily pollination syndrome. The species require the pollinator services to set fruit and seeds, and it suffers inbreeding depression if self-pollinated, so pollination among ramets decreases seed set, representing a cost of clonality. The selfing rate is high when plants are big or if clonality is frequent, as pollinators tend to visit nearby flowers increasing geitonogamy. The species has a mixed mating system with an outcrossing tendency, where delayed selfing (autogamy and geitonogamy) is a mechanism that ensure reproduction when outcrossing fails.