Factors influencing gene introgression into the allotetraploid Coffea arabica L. from its diploid relatives
Factors controlling gene introgression into cultivated arabica coffee (Coffea arabica L.) were investigated. Interspecific triploid hybrid plants between the tetraploid species C. arabica (2n = 44) and a diploid species (2n = 22), either Coffea canephora or Coffea eugenioides, were backcrossed to C. arabica (male parent). Flow cytometric analysis of the nuclear DNA content revealed that most of the BC1 individuals derived from triploid hybrids involving C. eugenioides were tetraploid or nearly tetraploid. Among the gametes produced by the interspecific triploid hybrids, those possessing approximately 22 chromosomes appeared strongly favored. The amount of introgression in BC1 individuals (21 and 43 for the BC1 progenies involving C. canephora and C. eugenioides, respectively) was estimated using species-specific microsatellite markers. A large number of introgressed markers was observed in all BC1 individuals. Nevertheless, while the frequency of introgressed markers seemed as expected, assuming random chromosome segregation and diploid gamete formation, in the BC1 derived from triploid hybrids involving C. canephora, this frequency appeared significantly lower in the BC1 derived from triploid hybrids involving C. eugenioides. Furthermore, the comparison of reciprocal progenies between C. arabica and triploid interspecific hybrids (C. arabica × C. canephora) used as male or female parent revealed a very strong effect of the backcross direction.Key words: irregular meiosis, coffee, reciprocal crosses, molecular marker, triploid hybrids.