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Title: Out-crossing between 'Bacon' pollinizers and adjacent 'Hass' avocado and the identification of two new avocado lethal mutants

item Schnell Ii, Raymond
item Tondo, Cecile
item Brown, James
item Ayala-Silva, Tomas
item BORRONE, JAMES - Oklahoma State University
item DAVENPORT, T - University Of Florida
item Kuhn, David

Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/1/2009
Publication Date: 10/9/2009
Citation: Schnell Ii, R.J., Tondo, C.L., Brown, J.S., Ayala Silva, T., Borrone, J.W., Davenport, T.L., Kuhn, D.N. 2009. Out-crossing between 'Bacon' pollinizers and adjacent 'Hass' avocado trees and the description of two new lethal mutants. HortScience. 44(6):1522-1526.

Interpretive Summary: Many plant species are capable of self pollination and cross pollination and the amount of self vs. cross pollination is governed by the nature of the species. Trees usually suffer from inbreeding depression when self pollinated and therefore most tree species are out-crossing. Avocado has developed a flowering process that promotes out-crossing between different individuals. Avocado cultivars can be characterized by flowering types as A and B with the A type flower opening as a female while the B type is opened as a male along with the reciprocal B type opening as a female while the A types are opened as a male. Out-crossing has been associated with increased yields in commercial groves and usually several cultivars are planted together. Hass is the most important cultivar world wide and growers receive a premium price for the fruit. Inter-planting with inferior cultivars reduces the number of Hass trees and the return to the farmer. A debate has occurred for many years among Avocado growers and scientists about the need for pollinator cultivars in commercial plantings as anecdotal evidences suggest that block plantings of single cultivars can yield as well as mixed plantings. This is the second study where we estimated outcrossing in commercial avocado plantings to determine the rate of self vs. cross pollination giving rise to commercial fruit. The orchard we sampled was in Santa Paula, California and consisted of pollinator rows of 'Bacon' and a commercial block planting of 'Hass'. We analyzed 919 fruit of 'Hass' and 850 seedlings of 'Bacon' using five fully informative (the parents don'tshare any alleles in common) microsatellite markers. We found that 75% of the seedlings of 'Hass' were fathered by 'Bacon' while 25% had resulted from 'Hass' self-pollination. For the 'Bacon' seedlings 45% were found to be fathered by 'Hass' while 55% resulted from 'Bacon' self-pollination. This is different from the results in Florida where the commercial cultivars 'Simmonds' and 'Tonnage' were found to be 75% and 95% outcrossed, respectively. The differences between the results from Florida and California may have resulted from different weather patterns and the genetic background of the cultivars.

Technical Abstract: Avocado (Persea americana Mill) has an unusual flowering mechanism, diurnally synchronous protogynous dichogamy, which promotes cross pollination among avocado genotypes. In commercial groves, which usually contain pollinizer rows adjacent to the more desirable commercial cultivars, the rate of out-crossing has been measured with variable results. Using microsatellite markers, we estimated out-crossing in a commercial California 'Hass' avocado orchard with adjacent 'Bacon' pollinizers. Seedlings grown from mature harvested fruit of both cultivars were genotyped with five fully informative microsatellite markers and their parentage determined. Among the 919 seedlings of 'Hass', 688 (75%) were hybrids with 'Bacon'; the remaining 231 (25%) seedlings were selfs of 'Hass'. Among the 850 seedlings of 'Bacon', 382 (45%) were hybrids with 'Hass' and the remaining 468 (55%) seedlings were selfs of 'Bacon'. The high out-crossing rate observed in the 'Hass' seedlings was expected as adjacent rows of opposite flowering types (A vs. B) are expected to outcross. However, the high selfing rate in 'Bacon' was unexpected. Two new lethal mutants were discovered among the selfed seedlings of 'Hass' and 'Bacon'. These were labeled 'spindly' and 'gnarly' and are similar in phenotype to mutants described in Arabidopsis and other crop species.