Self-Incompatibility in Abronia Macrocarpa, Large-Fruited Sand Verbena, (Nyctaginaceae)
Date
1996-05
Authors
Bazeer, Selvi K.M.
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Abstract
Abronia macrocarpa, large-fruited sand verbena, received a federal and state listing as endangered in 1988. The plant is a herbaceous perennial characterized by large anthocarps. A study of pollination biology by Williamson et al. ( 1994) revealed that A. macrocarpa does not set seed when experimentally self-pollinated. The plant is apparently reliant upon hawkmoths and noctuid moths to bring about cross-pollination for successful fruit set. This study examines the mechanisms, pre-fertilization or postfertilization barriers, that prevent successful seed set in self-pollinated flowers of A. macrocarpa. Plants were experimentally selfed and outcrossed. The resulting pollen germination and relative growth of pollen tubes were examined using fluorescent microscopy. Both the self- and outcross pollen readily adhered to the stigma and germinated with a 58-59% germination rate. But the growth of self-pollen tubes is arrested internally at the level of stigma with the formation of extensive callose deposits in the tubes. The cross-pollen tubes continue to grow down the style with regular callose plugs and reach the ovule within 72 hours following hand-pollination. These results indicated that a pre-fertilization barrier is operable. Two types of self-incompatibility mechanisms are known. The gametophytic self-incompatibility system is characterized by binucleate pollen grains with a long viability and wet stigmatic papillae. The incompatibility reaction is mediated by the genotype of the haploid pollen grain and occurs as the inhibition of pollen tube growth within the style. The sporophytic self incompatibility system is characterized by trinucleate pollen grains with a short viability and dry stigmas covered with a cuticle. The incompatibility reaction is mediated by the genotype of the diploid anther that produced the pollen and occurs as inhibition of pollen tube growth at the level of stigma. Stigmas were examined using light microscopy to determine the stigma type (dry or wet) and were chemically analyzed. A dry papillate type of stigma with no copious fluid secretions was observed in A. macrocarpa. Chemical analysis of the stigma revealed the presence of proteins, carbohydrates and lipids. Pollen was examined using light microscopy to study the development of the male gametophyte. The pollen is trinucleate, with two sperm cells and a vegetative nucleus, at the time of dehiscence. Stigmatic inhibition of pollen tubes, dry type stigma, and trinucleate pollen grains, correspond to the morphological and physical features of a sporophytic self incompatibility system. These data indicate that a sporophytic self-incompatibility system prevents seed set in self-pollinated flowers of A. macrocarpa.
Description
Keywords
verbena, pollen, nyctaginaceae, pollination, plants, self-incompatibility
Citation
Bazeer, S.K. (1996). Self-incompatibility in Abronia macrocarpa, large-fruited sand verbena, (Nyctaginaceae) (Unpublished thesis). Southwest Texas State University, San Marcos, Texas.