The genetic diversity of 255 taro (Colocasia esculenta) accessions from Vietnam, Thailand, Malaysia,Indonesia, the Philippines, Papua New Guinea and Vanuatu was studied using AFLPs. Three AFLP primer combinations generated a total of 465 scorable amplification products. The 255 accessions were grouped according to their country of origin, to their ploidy level (diploid or triploid) and to their habitat--cultivated or wild. Gene diversity within these groups and the genetic distance between these groups were computed. Dendrograms were constructed using UPGMA cluster analysis. In each country, the gene diversity within the groups of wild genotypes was the highest compared to the diploid and triploid cultivars groups. The highest gene diversity was observed for the wild group from Thailand (0.19), the lowest for the diploid cultivars group from Thailand(0.007). In Malaysia there was hardly any difference between the gene diversity of the cultivars and wild groups, 0.07 and 0.08, respectively. The genetic distances between the diploid cultivars groups ranges from 0.02 to 0.10, with the distance between the diploid accessions from Thailand and Malaysia being the highest. The genetic distances between the wild groups range from 0.05 to 0.07. First, a dendrogram was constructed with only the diploids cultivars from all countries. The accessions formed clusters largely according to the country from which they originated. Two major groups of clusters were revealed, one group assembling accessions from Asian countries and the other assembling accessions from the Pacific. Surprisingly, the group of diploid cultivars from Thailand clustered among the Pacific countries. Secondly,a dendrogram was constructed with diploid cultivated,triploid cultivated and wild accessions. Again the division of the accessions into an Asian and a Pacific gene pool is obvious. The presence of two gene pools for cultivated diploid taro has major implications for the breeding and conservation of germplasm.

A high level of resistance toGlobodera pallida pathotypes Pa2 and Pa3 exists inSolanum spegazzinii, a wild relative of potato (S. tuberosum ssp.tuberosum). Here we report the mapping of loci involved in quantitatively-inherited nematode resistance with the use of RFLPs. One major locus,Gpa, was mapped on chromosome 5 and two minor loci on chromosomes 4 and 7 ofS. spegazzinii. Additionally, the contribution of the susceptible parent to nematode resistance was determined. TheGpa locus was solely responsible for the high resistance level found in the segregating population. However, the RFLP marker closely linked to this resistance locus showed a distorted segregation, with a shortage of plants having the resistance linked allele. Our results indicate that a prediction of the genetic constitution of a quantitative trait based solely on phenotypic observations can lead to erroneous conclusions.

We report the identification and mapping of two quantitative trait loci (QTLs) of Solanum spegazzinii BGRC, accession 8218-15, involved in resistance to the potato cyst-nematode Globodera rostochiensis pathotype Ro1, by means of restriction fragment length polymorphisms (RFLPs). For this purpose we crossed a susceptible diploid S. tuberosum with the resistant S. spegazzinii, and tested the F1 population for resistance to the Ro1 pathotype. Since the F1 segregated for the resistance, the S. spegazzinii parent was concluded to be heterozygous at the nematode resistance loci. For the mapping of the resistance loci we made use of RFLP markers segregating for S. spegazzinii alleles in the F1. One hundred and seven RFLP markers were tested in combination with four different restriction enzymes; 29 of these displayed a heterozygous RFLP pattern within S. spegazzinii and were used for mapping. Analysis of variance (ANOVA) was applied to test the association of the RFLP patterns of these markers with nematode resistance. Two QTLs involved in disease resistance to Globodera rostochiensis pathotype Ro1 were identified and mapped to chromosomes 10 and 11 respectively.

In this paper we describe the reduced recombination and distorted segregation in an interspecific hybrid between Solanum tuberosum and Solanum spegazzinii. To study these phenomena, a cross was made between a (di)haploid S. tuberosum, used as a female parent, and a diploid wild potato species, S. spegazzinii, used as a male parent. Next, a backcross (BC) population was made with F1 genotype 38 that was backcrossed to S. tuberosum. In the backcross, S. tuberosum was used as the male parent. RFLP linkage maps were made using the F1 and the BC populations, yielding linkage maps of the interspecific hybrid, S. spegazzinii, and S. tuberosum from which male and female linkage maps could be constructed. The computer program JOINMAP was used to construct and combine the separate linkage maps. Subsequently, the separate linkage maps were compared with each other, and reduced recombination was observed in the linkage maps of the male S. tuberosum and the interspecific hybrid. The reason for this reduced recombination is discussed. Another common feature in linkage maps is the observation of distorted segregation. The distorted segregation of alleles from the interspecific hybrid was studied in more detail in the BC population. Most of the distortion was probably caused by gamete selection, but for 3 loci, on chromosomes 2, 3, and 4, we found evidence for the presence of a strong selection force acting at the zygote level against homozygous genotypes.

A backcross population, derived from the cross (S. tuberosumxS. spegazzinii)xS. tuberosum was used to map QTLs involved in nematode resistance, tuber yield and root development. Complete linkage maps were available for the interspecific hybrid parent as well as the S. tuberosum parent, and interval mapping for all traits was performed for both. Additionally, the intra- and inter-locus interactions of the QTLs were examined. The Gro1.2 locus, involved in resistance to G. rostochiensis pathotype Ro1, that was previously mapped in the S. tuberosumxS. spegazzinii F1 population, was located more precisely on chromosome 10. A new resistance locus, Gro1.4, also conferring resistance to G. rostochiensis pathotype Ro1, was found on chromosome 3. Different alleles of this locus originating from both parents contributed to the resistant phenotype, indicating multiallelism at this locus. No interlocus interactions were observed between these two resistance loci. For resistance to G. pallida no QTLs were detected. One minor QTL involved in tuber yield was located on chromosome 4. Two QTLs involved in root development and having large effects were mapped on chromosomes 2 and 6 and an epistatic interaction was found between these two loci.