Evolution of gametophytic apomixis in flowering plants: an alternative model from Maloid Rosaceae

Abstract

Gametophytic apomixis, asexual reproduction involving megagametophytes, occurs in many flowering-plant families and as several variant mechanisms. Developmental destabilization of sexual reproduction as a result of hybridization and/or polyploidy appears to be a general trigger for its evolution, but the evidence is complicated by ploidy-level changes and hybridization occurring with facultative apomixis. The repeated origins of polyploid apomictic complexes in the palaeopolyploid Maloid Rosaceae suggest a new model of evolutionary transitions that may have wider applicability. Two conjectures are fundamental to this model: (1) that as previously suggested by Rutishauser, like many sexual flowering plants the polyploid apomicts require maternal-paternal balance in the second fertilization event that gives rise to the endosperm, and (2) that the observed variation in endosperm ploidy levels relates less to flexibility late in development than to the known variation in developmental origin of the megagametophyte between mechanisms loosely categorized as diplospory and apospory. The model suggests explanations for the relative frequencies of apospory and diplospory, and for the wide but incomplete associations of apospory with a pollination requirement (pseudogamy) and of diplospory with autonomous development of the endosperm. It is suggested that pollination from other taxa may provide some adaptive advantage to pseudogamous apospory.

Genetic variability and morphology of tri- and tetraploid members of the Sorbus aria complex in northern Bavaria

“…This could be either the result of different independent polyploidization events between different individuals of the same taxa (multiple origin, Nelson-Jones et al 2002), or due to backcrossing with S. aria s. str. Backcrossing yields mainly triploid individuals; tetraploids could arise by the fertilization of unreduced triploid egg cells with haploid pollen (triploid bridge; Talent 2009. The Northern Franconian unique tetraploid genotypes do not have specific and indicative alleles of the Franconian triploids (e.g.…”

Cytotype stability, facultative apomixis and geographical parthenogenesis in Ranunculus kuepferi (Ranunculaceae)

“…Stable triploidy is also widespread in dandelions ( Taraxacum officinale group; Van Dijk, 2003). In Asteraceae, apomixis may actually arise at the triploid level because of their mode of embryo sac development and endosperm formation (Talent, 2009). In Ranunculus kuepferi , the present results rather suggest that apomixis originated in tetraploid cytotypes, because of the existence of rare sexual tetraploids and a frequent uncoupling of apomeiosis and parthenogenesis on this ploidy level.…”

“…This may not only have negative effects on the fitness of the embryo, but also on endosperm formation. In the endosperm of flowering plants, a ratio of two maternal and one paternal copies of the genome are optimal for development, probably because of genomic imprinting; deviations from this ratio are sometimes tolerated, but often result in disturbances in seed formation (Vinkenoog et al , 2003, Spielmann et al , 2003; Talent, 2009). Since the majority of apomictic plants are pseudogamous and need pollen for endosperm fertilization, interploidal crosses pose a problem because they cause endosperm imbalance and potentially seed abortion.…”

Cytogenetics and Reproductive Biology of <i>Bixa orellana</i> L. (Bixaceae)

“…This result indicates that there is no adventive embryony in B. orellana, which would produce apomictic embryos by autonomous development of somatic cells (Talent 2009). However, there are other processes that might produce parthenocarpic fruits and could not be investigated by emasculating tests, such as pseudogamous diplospory and apospory (Bhat et al 2005).…”