Search

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 phytophthora root rot) 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 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.

We previously analyzed genetic diversity and population structure of eight cultivars within Florida coconut (Cocos nucifera L.) germplasm using 15 microsatellite (simple sequence repeat, SSR) markers. Here we report on the analysis of the same genotypes using 13 markers derived from WRKY sequences containing single nucleotide polymorphisms (SNP) and one microsatellite. WRKY transcription factors are unique to plants and are involved in numerous vital processes including disease resistance. Our objective was to determine the value of this type of gene to assess thegenetic diversity of this species. Overall, the WRKY results are similar to thosewith microsatellite markers. Despite the lower number of alleles identified with the WRKY-derived markers (37 versus 67 total alleles identified with the SSR markers), individuals of cultivars ‘Red Malayan Dwarf’, ‘Fiji Dwarf’ (‘Niu Leka’) and ‘Red Spicata’ were clearly clustered, as in the previous analysis. However, individuals of ‘Green Malayan Dwarf’ and ‘Yellow Malayan Dwarf’ cultivars were resolved with other varieties, perhaps due to selective forces operating on these functional genes. Most samples of the ‘Atlantic Tall’ and ‘Panama Tall’ cultivars clustered together as in the previous microsatellite study. We discuss the utility of WRKY-derived markers for assessing the genetic diversity of coconut, and their possible value in the study of other species. Published by Elsevier B.V.

Twenty-six jackfruit accessions, one interspecific hybrid, champedak, and one breadfruit accession were analyzed using amplified fragment length polymorphic (AFLP) markers to determine the degree of genetic diversity within the Fairchild Tropical Garden (FTG) germplasm collection. Of the 30 primer pairs evaluated, 12 were identified for collection screening based on number and quality of polymorphic fragments. A total of 187 AFLP markers were scored using the 12 primer pairs, 92 (49.2%) being polymorphic. All accessions could be uniquely identified using the 12 primer pairs. Among the jackfruit accessions, similarity coefficients ranged from 0.567 to 0.950; the accessions also shared a large number of monomorphic fragments (54.9%). Cluster analysis and principal component analysis (PCA) grouped all of the jackfruit accessions with south-east Asian origins into one major cluster with little bootstrap support for groupings within the cluster. The Indian accessions were grouped in a different cluster, as did the hybrid and the breadfruit accession. The AFLP marker based analysis indicates that limited genetic diversity exists within this collection. These observations are in agreement with the phenotypic evaluation and suggest that new accessions be obtained from the center of origin for the species.

Eighteen cultivars of cocoyam (Xanthosoma spp.) and two cultivars of taro (Colocasia esculenta (L.) Schott) from the USDA/ARS germplasm collection were evaluated for genetic relatedness using RAPD data. Seven random primers generated 40 RAPD loci. Of the 18 cultivars screened, 11 (61%) were identical at all RAPD loci evaluated. A similarity matrix was constructed on the basis of the presence or absence of bands. Among cocoyam cultivars the genetic similarity ranged from 0.86 to 0.97 with a mean of 0.91. Cluster analysis identified two main clusters with some unexpected groupings. These data indicate that very little genetic variation exists within the accessions used in this study and that this Xanthosoma spp. collection is of limited value as a genetic resource.