Genetic analysis of apomixis in Citrus and Poncirus by molecular markers

Propagation of citrus rootstocks depends upon the production of clonal plan ts from nucellar seedlings. This makes apomixis one of the host important t raits in breeding programs for citrus rootstocks. The genetic control of ap omixis was studied in a 50-tree progeny derived from the cross C. volkameri anaxP. trifoliata using 69 molecular markers and bulked segregant analysis. The proportion of nucellar seedlings was estimated by isoenzymatic analysi s of 25 seedlings per tree for 2 consecutive years. The type of embryony (p olyembryonic versus monoembryonic seeds) was also determined for fruit-yiel ding trees. Separate genetic maps for each parental species were developed. The integration and comparison of these maps could be accomplished using c ommon 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 involv ing differences in the C-methylation pattern, have been observed, especiall y for the pollen parent. Analysis of quantitative trait loci (QTLs) reveale d 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 alle le effects were present, even in the same linkage group. One of the markers associated to apomixis (Apo2) is also associated to embryony type. Therefo re, the genetic control of apomictic reproduction found in citrus (nucellar embryony) is quite complex compared to what has been reported for gametoph ytic apomixis. Molecular markers linked to QTLs governing apomixis will be useful to assist selection of future apomictic rootstocks for citrus variet ies.