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Title: Microsatellite markers reveal low breeding system efficacy and pollen contamination can limit production of full-sib avocado progeny

item Brown, James
item Tondo, Cecile
item Schnell Ii, Raymond

Submitted to: Scientia Horticulturae
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/3/2008
Publication Date: 1/1/2009
Citation: Violi, H.A., Brown, J.S., Tondo, C.L., Borrone, J.W., Ploetz, R., Schnell Ii, R.J. 2009. Microsatellite markers reveal low breeding system efficacy and pollen contamination can limit production of full-sib avocado progeny. Scientia Horticultureae. doi:10.1016/j.scienta.2008.11.011

Interpretive Summary: Avocado is plagued by a serious root rot disease caused by the fungus Phytophthora cinnamomi. The USDA-ARS station in Miami has an ongoing selection program to select rootstocks of avocado that are resistant to the disease and elite parents that impart resistance to their progeny have been identified. Controlled matings are difficult to make because of the large number of flowers that abort every year. To increase the number of progeny from elite parent matings complementary cultivars were juxtaposed by grafting on mature rootstocks in 2004. In 2006 progeny were harvested from 27 of the grafted scions and 325 progeny were obtained. High rates of outcrossing were achieved but unfortunately only 42% of the seedlings were from crosses with elite parents. The remaining seedlings were outcrossed with non-elite parents (offtypes) that were in adjacent growing areas. Offtype and selfed progeny were significantly less tolerant to PRR than were the progeny derived from elite parents. Maternal effects were also found to be significant for PRR resistance.

Technical Abstract: Phytophthora cinnamomi causes a severe root rot in avocado, Persea americana. Breeding tolerant rootstocks is thought to be the most promising method for phytophthora root rot disease control but breeding avocado is challenging. The avocado flowering syndrome (synchronous protogynous dichogamy), combined with high flowering and low fruit set, render controlled pollination exceedingly difficult. Juxtaposing complementary flowering types of elite parent cultivars (cultivars that produce progeny with tolerance to PRR) was performed in an effort to increase the number of full sib progeny for elite maternal parents and, hypothetically, the number of phytophthora root rot tolerant progeny. Although high outcrossing rates were achieved (estimated ~93%), the majority of progeny had a non-elite paternal parent (56% of progeny were offtypes) implying maternal trees were pollinated by non-elite distant trees. Among progeny that could be confidently genotyped, a high number of cross types were detected (33). Contrary to our hypothesis, a significant portion of the progeny were the result of crosses between like, and not complementary, flowering types. The spatial distribution of productive trees and grafts helped to explain these data, as productive grafts were directly adjacent to grafts of the same flowering type more often than that of the complementary flowering type. Selfed progeny were significantly less tolerant to phytophthora root rot than outcrossed progeny. Progeny resulting from crosses between an elite maternal parent and non-elite pollen donor (offtypes) were less tolerant than full-sib progeny resulting from crosses between elite parents. Maternal effects may interfere with the identifying truly disease tolerant selections. Thus, to reduce maternal effects and non-elite pollen donor contamination, removal of seedling cotyledons before screening for disease tolerance and better isolation of elite parent trees and windbreaks may improve breeding efficacy. This study also demonstrates the usefulness of microsatellite markers in parentage analysis where a high proportion of the putative parents are closely related.