Genetic mechanisms of apomixis

Abstract

The introduction of apomixis to crops would allow desirable genotypes to be propagated while preventing undesirable gene flow, but so far there has been little success in transferring this trait from a natural apomict to another species. One explanation is the sensitivity of endosperm to changes in relative maternal and paternal contribution owing to parental imprinting, an epigenetic system of transcriptional regulation by which some genes are expressed from only the maternally or paternally contributed allele. In sexual species, endosperm typically requires a ratio of two maternal genomes to one paternal genome for normal development, but this ratio is often altered in apomicts, suggesting that the imprinting system is altered as well. We present evidence that modification of DNA methylation is one mechanism by which the imprinting system could be altered to allow endosperm development in apomicts. Another feature of natural apomixis is the modification of the normal fertilization programme. Sexual reproduction uses both sperm from each pollen grain, but pseudogamous apomicts, which require a sexual endosperm to support the asexual embryo, often use just one. We present evidence that multiple fertilization of the central cell is possible in Arabidopsis thaliana , suggesting that pseudogamous apomicts may also need to acquire a mechanism for preventing more than one sperm from contributing to the endosperm. We conclude that strategies to transfer apomixis to crop species should take account of endosperm development and particularly its sensitivity to parental imprinting, as well as the mechanism of fertilization.

Mendelian segregation for two-factor apomixis in Erigeron annuus (Asteraceae)

“…For most apomicts that are not Asteraceae, pleiotropy of this type does not occur, and fertilization of the central cell is required for endosperm development. There is a rich literature on endosperm genome balance in sexual and apomictic plants (Haig and Westoby, 1991;Quarin, 1999;Spielman et al, 2003).…”

Charecterization of thePHERES1Orthologs in the ApomictBoechera

“…The fertilization of the two unreduced polar nuclei with a reduced sperm results in a pentaploid endosperm with a 4m:1p ratio which is usually disruptive. However, as it was reported for BH1 plants (15), in order to allow a 4m:1p ratio in the endosperm, these plants likely require modifications in their imprinting systems (30).…”

“…Apomicts whether required fertilization for endosperm development or not, need to cope with parental genomic imbalance, as they produce an unreduced embryo sac. Various modifications to restore the normal balance were suggested, including a modification of the imprinting system (30). Apomictic Boechera (Brassicaceae) species are attractive model species to study the genetic imprinting in apomicts since they are close relatives of Arabidopsis thaliana (8,9,25,26,28).…”

Reproductive characteristics of citrus rootstocks grown under greenhouse and field environments

“…Even considering that these plants reproduce by apomixis (Cameron andSoost 1969, Koltunow 1993), viable pollen grains are necessary for fertilization and endosperm development of the sexual embryo, therefore assuring the essential nutrients and hormones for nucellar embryo growth and viable seed formation (Koltunow 1993, Davies and Albrigo 1994, Spielman et al 2003, Machado et al 2005. Low percentage of viable pollen and, consequently, low seed production has been observed in orange, mandarin and lime by Moreira and Gurgel (1941) and Sellito-Boaventura and Pio (1989).…”