Pollen tube Growth and Self-Incompatibility in Heuchera micrantha var. diversifolia (Saxifragaceae).
A crossing study and an analysis of pollen tube growth were conducted in diploid and autotetraploid Heuchera micrantha var. diversifolia to distinguish between the possible mechanisms that could promote the high outcrossing rates observed and that could also result in the absence of fruit set following self-fertilization. The crossing study indicated that no fruit set occurred after self-pollination, whereas fruit set occurred in all of the hand-pollinated outcrosses. After 4 d, the self-pollinated flowers shriveled and abscised. Pollen tube growth following hand pollination was assessed in selfed and outcrossed flowers using fluorescence microscopy. The self-pollinated flowers exhibited far fewer pollen tubes than did the outcrossed flowers. Furthermore, in self-pollinated flowers, some of the pollen tubes extended into the style; fewer than one-half of the pollen tubes reached the base of the style and still fewer reached the ovules. The variable length of pollen tube growth, the uniform timing of floral abortion after self-pollination, and the absence of variability among individuals in the level of fruit set following self-pollination are all consistent with a system that lies somewhere between classic gametophytic self-incompatibility and late-acting self-incompatibility as the mechanism that is most likely operating in H. micrantha var. diversifolia. A similar "nonstandard" system may be present in other Saxifragaceae, such as Tolmiea and Lithophragma, as well as in Ribes, the sister group of Saxifragaceae. Our data also indicate that ploidal level (diploid vs. autotetraploid) has no influence on the extent or mechanism of self-incompatibiltiy in autopolyploid H. micrantha var. diversifolia.