Pollen tube growth in a living rock cactus, Ariocarpus kotschoubeyanus (Cactaceae): A case of partial-self-incompatibility?

Background/Question/Methods . It is acknowledge that inbreeding depression is a consequence of selfing, so mechanisms that avoid self-fertilization have evolved in angiosperms. However, selfing is a viable strategy to ensure progeny under harsh environmental conditions. Self-incompatibility (SI) is the most effective mechanism that precludes self-fertilization, since growth of self pollen tubes is inhibited. Pioneer research of SI in Cactaceae report a single locus, multiallelic, gamethophytic system. We attempted to determine the presence of SI in Ariocarpus kotschoubeyanus, an endemic and threatened species of the Chihuahuan Desert. Previous pollination experiments suggest that A. kotschoubeyanus has a mixed mating system, with ca. 30% of fruit set from forced self-pollination. We conducted a 3 x 2 experimental design, with three pollination treatments on flowers of different age to evaluate the presence of SI: forced self-pollination (SP), manual cross-pollination (CR) and control (C). Since flowers live two days, we collected flowers in FAA after the first day of life (d1) and after the second day of life (d2). Pistils were softened with NaOH and stained with aniline blue. Stained pistils were prepared in squash slides and observed under a UV microscope to assess pollen tube growth.

Results/Conclusions . In both SPd1 and SPd2 treatments, ca. 15% of pistils showed pollen tubes at the ovary, whereas 57% of SPd1 and 39% of SPd2 treatments stopped growing at mid-style. Pollen tubes were seen at the ovary in ca. 7% from CRd2, and in 54% of Cd2 pistils. The low percentage of CR pistils with pollen tubes in the style resulted from an inadequate pollination (i. e. damage to the flower or insufficient pollen), or from the frequency of S-alleles in the population that limits possible cross-pollinations. The Cd2 treatment showed that in 54% of the cases an effective fertilization takes at least two days, so damage to the pistil in open flowers (by florivory) means a direct effect in plant fertility. Together with previous pollination experiments, these observations suggest a partial SI system where functionally S-alleles exist, with some compatible individuals. However, the failure in several pollination treatments due to the time of stigma receptivity and the amount and viability of pollen loads, should be considered in order to contrast the potential success of outcrossed pollen and the frequency of S-alleles.