Is supernumerary chromatin involved in gametophytic apomixis of polyploid plants?

Abstract

Gametophytic apomixis, or unreduced embryo sac development that results in asexual reproduction through seeds, occurs in several families of angiosperms and must be polyphyletic in origin. The molecular mechanisms underlying gametophytic apomixis have not been discovered and are the subject of intense investigation. A common feature of almost all apomicts is their polyploid nature. From genetic mapping studies in both monocots and dicots, there is low genetic recombination associated with a single (rarely two), dominant locus for either aposporous or diplosporous embryo sac formation. In Pennisetum squamulatum and Cenchrus ciliaris, some DNA sequences mapping to the apospory locus are unique to apomictic genotypes and apparently hemizygous. This sequence divergence at the apomixis locus could be a consequence of genome rearrangements and isolation from genetic recombination, both of which may have contributed to the definition of a chromosomal region as supernumerary. The possible involvement of supernumerary chromatin, formed as a result of interspecific hybridization, in the origin of apomixis, is explored here.

Understanding Apomixis: Recent Advances and Remaining Conundrums

“…Whether this is universally true, and whether it can be used to harness apomixis in crop species, are questions remaining to be answered. Roche et al (2001b) provided a refinement on Carman's hypothesis, suggesting that supernumerary chromatin may be the principal driver in this process. A hybrid origin, segmental allopolyploidy, and the activity of reproductive drivers all are reported characteristics of supernumerary chromatin biology (McVean, 1995).…”

Molecular cytogenetics and DNA sequence analysis of an apomixis-linked BAC in Paspalum simplex reveal a non pericentromere location and partial microcolinearity with rice

“…The presence of sequences that are strictly apomixis-specific (i.e. sequences that do not hybridise to DNA of sexual relatives nor to that of the rest of the genome of apomictic genotypes) may suggest the existence of apomixis-specific genetic factors and/or a possible role of supernumerary chromatin in the control of apomictic reproduction (Roche et al 2001). Alternatively a functional role of the apomixis-specific sequences is suggested: a proposed model hypothesizes that these regions act as a sink to sequester proteins that regulate genes involved in megagametogenesis thereby inducing mis-regulation of sexual process (Koltunow and Grossniklaus 2003).…”

B Carrier Genome in Soybean (Glycine max)

“…Various mechanisms have been proposed to explain the origin and maintenance of B-chromosomes (Cabrero et al 2003, Medeiros et al 2006, Roche et al 2001. Traditionally, these chromosomes are believed to be derived from autosomes (Jones and Rees 1982) which may either be euchromatic or heterochromatic in nature i.e.…”