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Recent enhancement of the pool of known molecular markers for avocado has allowed the construction of the first moderately dense genetic map for this species. Over 300 SSR markers have been characterized and 163 of these were used to construct a map from the reciprocal cross of two Florida cultivars 'Simmonds' and 'Tonnage'. One hundred thirty-five primer pairs amplified 163 usable loci with 20 primer pairs amplifying more than one locus. 'Tonnage' was heterozygous for 152 (93%) loci, whereas 'Simmonds' was heterozygous for 64 (39%). Null alleles were identified at several loci. Linkage maps were produced for both reciprocal crosses and combined to generate a composite linkage map for the F1 population of 715 individuals. The composite map contains 12 linkage groups. Linkage groups ranged in size from 157.3 cM (LG2) to 2.4 cM (LG12) and the number of loci mapped per group ranged from 29 (LG1) to two (LG12). The total map length was 1,087.4 cM. Only seven markers were observed to have segregation distortion (α ≤ 0.05) across both sub-composite (reciprocal) maps. Phenotypic data from traits of horticultural interest are currently being collected on this population with the ultimate goal of identifying useful quantitative trait loci and the development of a marker-assisted selection program.

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.

Theobroma cacao L. expressed sequence tags (ESTs) were converted into useful genetic markers for fingerprinting individuals and genetic linkage mapping. Primers were designed to microsatellite-containing ESTs. Twenty-two T. cacao accessions, parents of various mapping populations segregating for disease resistance and crop yield characteristics, were tested. Twenty-seven informative loci were discovered with 26 primer pairs. The number of detected alleles ranged from two to 11 and averaged 4.4 per locus. All 27 markers could be mapped into at least one of the existing F 1 or F 2 populations segregating for agronomically important traits.

Expressed sequence tags for Persea americana Mill. were investigated to expand upon the number of informative microsatellite markers available for avocado. Seventy informative loci were discovered using 24 P. americana var. americana Mill. accessions. The number of alleles detected ranged from two to 17 and averaged 7.1 alleles per locus. These primers successfully amplified products in different varieties of P. americana, hybrids and a related species, Persea schiedeana. These primers will be useful for characterizing germplasm, determining genetic relationships of cultivated accessions, and for marker-assisted development of root rot-tolerant P. americana var. americana rootstock material.

There is currently an international effort in improving disease resistance and crop yield in Theobroma cacao L., an economically important crop of the tropics, using marker-assisted selection for breeding. We are developing molecular genetic markers focusing upon gene families involved with disease resistance. One such family is the WRKY proteins, which are plant-specific transcriptional factors associated with regulating defense responses to both abiotic and biotic stresses. Degenerate PCR primers were designed to the highly conserved DNA-binding domain and other conserved motifs of group I and group II, subgroups a-c, WRKY genes. Sixteen individual WRKY fragments were isolated from a mixture of T. cacao DNA using one pair of primers. Of the 16 WRKY loci investigated, seven contained single nucleotide polymorphisms within the intron as detected by sequence comparison of the PCR products. Four of these were successfully converted into molecular markers and mapped in an F2 population by capillary electrophoresis-single strand conformation polymorphism analysis. This is the first report of a pair of degenerate primers amplifying WRKY loci directly from genomic DNA and demonstrates a simple method for developing useful genetic markers from members of a large gene family.