The reciprocal effect of two avocado (Persea americana Mill.) cultivars-Ardith and Ettinger-on outcrossing rate and yield was studied in several orchards in Israel. Multilocus estimates of outcrossing rates were made using the isozyme loci Mdh-1 (malate dehydrogenase) and Aat-l (aspartate aminotransferase) for 'Ettinger' progeny and Lap-2 (leucine aminopeptidase), Pgm-1 (phosphoglucomutase) and Tpi-1 (triosephosphate isomerase) for 'Ardith' progeny. When the two cultivars were in close proximity, estimated yields ranged from 10 to 20 t.ha(-1) and outcrossing rates ranged from 0.71 to 0.89 and from 0.87 to 0.90 for 'Ettinger' and 'Ardith', respectively. The effect of 'Ettinger' as a pollenizer was not restricted to adjacent 'Ardith' trees; it also reached more distant 'Ardith' trees. Thus, outcrossing rate in 'Ardith' was 0.82 at a distance of 30 m from 'Ettinger' in one orchard and 0.91 at a distance of 36 m in another orchard. These results confirm previous observations that 'Ettinger' is a highly potent pollenizer. Outcrossing rates in 'Ardith' and 'Ettinger' were found to increase from the young fruitlet stage to that of mature fruit. These findings provide evidence for selective abscission of selfed fruitlets. In addition, parentage analysis of abscised versus retained 'Ardith' fruit showed that 'Ardith' selfed fruit abscised at a much higher rate than outcrossed ones. The survival advantage of outcrossed fruit is probably related to the fact that selfed progeny have less-vigorous embryos than outcrossed progeny due to inbreeding depression.
