INTERSPECIFIC COMPATIBILITY AMONG 15 LEUCAENA SPECIES (LEGUMINOSAE, MIMOSOIDEAE) VIA ARTIFICIAL HYBRIDIZATIONS

Leucaenas are fast-growing neotropical trees and shrubs used as forage and fuelwood. This study tested the seed cross-compatibilities among 15 of the 16 well-defined species of the paleopolyploid genus Leucaena Bentham (Leguminosae: Mimosoideae) to determine if genes of interest could be transferred between species. Twelve diploid and four tetraplo id taxa were artificially hybridized in 118 of all 120 possible two-wa y combinations and 31 of 32 possible self- and intraspecific matings, with 58,218 floret pollinations. Ninety-one combinations (77%) produce d well-filled seed, and 73 of these combinations were grown and valida ted in the field. The qualitative crossability within ploidy levels (d iploid, triploid, tetraploid) was 78%, 75%, and 100%, respectively, wh ereas quantitative crossability (relative to intraspecific matings) of successful matings within identical ploidy levels was 32%, 11%, and 7 3%, respectively. One-third (34%) of the 114 interspecific combination s tested in both directions exhibited weak unilateral incompatibility (UI), due, in part, to reciprocal differences in stylar length. Other mechanisms for UI (embryo-endosperm imbalance, sex-specific use of sel f-compatible parents) were ruled out. Wide hybridization appears to be ''buffered'' by polyploidy in the genus.