Genetic analysis of apomixis in Citrus and Poncirus by molecular markers

Abstract

Propagation of citrus rootstocks depends upon the production of clonal plants from nucellar seedlings. This makes apomixis one of the host important traits in breeding programs for citrus rootstocks. The genetic control of apomixis was studied in a 50-tree progeny derived from the cross C. volkameriana×P. trifoliata using 69 molecular markers and bulked segregant analysis. The proportion of nucellar seedlings was estimated by isoenzymatic analysis of 25 seedlings per tree for 2 consecutive years. The type of embryony (polyembryonic versus monoembryonic seeds) was also determined for fruit-yielding trees. Separate genetic maps for each parental species were developed. The integration and comparison of these maps could be accomplished using common multiallelic segregant loci. Differences in gene synteny between the two species-specific genetic maps were shown. Important distortions in the segregation of markers at several genomic regions, some of them also involving differences in the C-methylation pattern, have been observed, especially for the pollen parent. Analysis of quantitative trait loci (QTLs) revealed the presence of six genomic positions (two in P. trifoliata and four in C. volkameriana) contributing individually up to 24% of the total variation for apomixis. Within the same species, QTLs with positive and negative allele effects were present, even in the same linkage group. One of the markers associated to apomixis (Apo2) is also associated to embryony type. Therefore, the genetic control of apomictic reproduction found in citrus (nucellar embryony) is quite complex compared to what has been reported for gametophytic apomixis. Molecular markers linked to QTLs governing apomixis will be useful to assist selection of future apomictic rootstocks for citrus varieties.

The Genetic Control of Apomixis: Asexual Seed Formation

“…The sexually derived embryo may or may not mature or germinate (Koltunow et al 1995;Koltunow et al 1996). Sporophytic apomixis has not been extensively studied at the molecular level; however, it appears genetically complex (García et al 1999). The remainder of this review focuses on gametophytic apomixis.…”

Polyembryony and identification of Volkamerian lemon zygotic and nucellar seedlings using RAPD

“…For C. volkameriana rootstock, García et al (1999) reported only 22% zygotic seedlings from seeds derived from open pollination, and 60% of zygotic seedlings were obtained after pollination with Poncious trifoliata. The authors suggest that when the zygotic embryo is a hybrid, it may be more vigorous, and hence compete better with nucellar embryos, whereas zygotic embryos produced by self-pollination are less vigorous and may not be competitive with nucellar ones.…”

Stone Characterization, Media Analysis and its Influence on Polyembryonic Rootstocks Germination of Mango (Mangifera indica L.)

“…The zygotic seedlings as well as nucellar seedlings are used as rootstocks (Bally, 2006). The Nucellar seedlings are identical to its parent plant (Xiang and Roose, 1988;Garcia et al, 1999;Ruiz et al, 2000). Rao et al, 2008, reported that nucellar seedlings are a true to type and which maintain the genetic homogeneity of rootstocks.…”