Abstract
Flowering plants often produce ovules that do not develop into seeds. Lack of pollination, insufficient resources, environmental constraints and incompatibility reactions are often considered to be causal factors. Pre-pollination and post-fertilization developmental irregularities (DI) are less commonly implicated as causal factors, usually because information on them is limited. The results of a detailed developmental assessment of sexual reproduction inB. umbellatususing self- and cross-pollinated glasshouse and open-pollinated field-grown plants, reveal a high degree of pre-pollination and post-fertilization DI. Pre-pollination DI account for a large fraction of the species' low reproductive potential. Post-fertilization DI represent another crucial reduction in the species' reproductive output. Seed set inB. umbellatusis less than 1%. In the female reproductive component, these DI include embryo sacs with hypertrophied or highly vacuolate eggs, collapsed synergids, collapsed egg apparati and embryo sacs that are empty, uninucleate or with disorganized mass of nuclei; there are also ovules with no embryo sacs. Post-fertilization DI include zygotes that are hypertrophied, highly vacuolate and collapsed, and malformed embryos. The lack of an obvious adaptive significance to the array of DI inB. umbellatussuggests that these are unlikely to be due to self-incompatibility reactions, and physiological or environmental constraints. SinceB. umbellatusis a long-lived species, it is highly possible that disadvantageous mutations have accumulated in its clonal lineages causing a considerable load in terms of faulty meiosis resulting in defective meiotic products, a series of DI, and therefore, limited sexual reproduction.