Outcrossing in Florida Avocados as Measured Using Microsatellite Markers

Abstract

<jats:p>Avocado (<jats:italic>Persea americana</jats:italic> Mill.) possesses a unique flowering mechanism, thought to promote out-crossing, in which the male and female parts of the perfect flower function at different time periods. Cultivars are classified as Flowering Type A, where flowers are functionally female the morning of one day and functionally male the afternoon of the next day, or Flowering Type B, where flowers are functionally female in the afternoon and functionally male the next morning. Avocado growers typically interplant cultivars of opposite flowering types to maximize yield. Recently, it has been hypothesized that 90% to 95% of avocado flowers are self-pollinated in southern Florida. However, this hypothesis does not address whether mature, marketable avocado fruit in Florida are the result of outcrossing. To determine whether avocado fruit in southern Florida result from self-pollination or outcrossing, fruit were harvested from a commercial orchard in Miami-Dade County, Florida, from a block consisting of two cultivars, Simmonds (Flowering Type A) and Tonnage (Flowering Type B), interplanted in approximately equal numbers. Seeds were germinated and the resulting progeny were genotyped using eight fully informative, microsatellite markers. Seventy-four percent of the ‘Simmonds’ progeny and 96% of the ‘Tonnage’ progeny were judged to be the result of cross-pollination, with an estimated overall outcrossing rate of 63% to 85% within this particular block of the orchard. Seedlings judged to be the result of cross-pollinations between ‘Simmonds’ and ‘Tonnage’ are being maintained at the U.S. Department of Agriculture-Agricultural Research Service, Subtropical Horticulture Research Station and are being evaluated for segregation of important agronomic and horticultural traits.</jats:p>

An EST-SSR-based linkage map for Persea americana Mill. (avocado)

“…Some primer pairs produced complex electrophoretic patterns, in which allelic assignments could not be made with confidence in terms of individual loci, and these markers were not used. Null alleles were identified for several markers, as reported previously in avocado (Sharon et al 1997;Ashworth et al 2004;Borrone et al 2007Borrone et al , 2008 and these markers were retained. In some instances, an amplification product was present in one parent and entirely absent in the other, but clearly segregated in the progeny in a 1:1 ratio.…”

“…americana cv. Simmonds is described in Borrone et al (2008). Linkage maps were constructed from 715 F 1 individuals: 456 'Tonnage'×'Simmonds' progeny and 259 'Simmonds'×'Tonnage' progeny.…”

“…Avocado has been proposed to be the result of an ancient polyploid event (Chanderbali et al 2008). However, evidence from analyses of germplasm collections and seedling populations using simple sequence repeat (SSR) markers have demonstrated diploid inheritance for most markers although some primer pairs have amplified more than one locus (Schnell et al 2003;Borrone et al 2007Borrone et al , 2008.…”

“…Recently, a large population of seedlings from a commercial grove inter-planted with two cultivars of opposite flowering types, 'Tonnage' (Type B) and 'Simmonds' (Type A), in approximately equal numbers was screened to determine the out-crossing rate in avocado under south Florida conditions (Borrone et al 2008). Eight fully informative SSR markers identified 870 seedlings as progeny of a reciprocal cross between 'Tonnage' and 'Simmonds'.…”

Genetic diversity analysis of avocado (Persea americana Miller) rootstocks selected under greenhouse conditions for tolerance to phytophthora root rot caused by Phytophthora cinnamomi

“…Therefore, it is impossible to distinguish between putative selfing and outcrossing based on how many markers are shared by any two genotypes. Persea americana possesses a unique flowering mechanism termed diurnally synchronous protogynous dichogamy where male and female structures of the flowers function at different time periods (Davenport 1986), which is thought to have evolved to promote outcrossing within the species (Borrone et al 2008). Most studies utilizing various molecular markers have found some level of selfing in P. americana but the rates vary according to the type of marker used, location, design of the orchard, developmental stage of the fruit sampled, and the cultivars investigated (Schnell et al 2009).…”