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.