Myanmar mango landraces reveal genetic uniqueness over common cultivars from Florida, India, and Southeast Asia.

Mango (Mangifera indica) is believed to have evolved in a large area spanning northeastern India, Bangladesh, and northwestern Myanmar. We compared the genetic structure of mango accessions from Myanmar with that of mango accessions from Florida, India, and Southeast Asia with 11 SSR markers. The Myanmar accessions exhibited considerable genetic diversity (unbiased heterozygosity, UHe = 0.698) and a high number of private alleles. Despite the low degree of genetic differentiation among accessions (global Fst, tetha = 0.123), Myanmar's accessions were distinguishable from mango accessions from Florida, India, and Southeast Asia in a principal coordinates plot. Genetic differentiation of the Myanmar accessions from other groups was also observed in a Bayesian cluster analysis. No population structure among Myanmar accessions was revealed by a neighbor-joining tree. Our results revealed a broad genetic background and genetic distinctiveness of mango in Myanmar. We discuss the implications for diversification mechanisms based on the embryo type characteristics and provide recommendations for conservation efforts.

Assessment of cytochrome P450 sequences offers a useful tool for determining genetic diversity in higher plant species.

To investigate and develop new genetic tools for assessing genome-wide diversity in higher plant-species, polymorphisms of gene analogues of mammalian cytochrome P450 mono-oxygenases were studied. Data mining on Arabidopsis thaliana indicated that a small number of primer-sets derived from P450 genes could provide universal tools for the assessment of genome-wide genetic diversity in diverse plant species that do not have relevant genetic markers, or for which, there is no prior inheritance knowledge of inheritance traits. Results from PCR amplification of 51 plant species from 28 taxonomic families using P450 gene-primer sets suggested that there were at least several mammalian P450 gene mammalian-analogues in plants. Intra- and inter- specific variations were demonstrated following PCR amplifications of P450 analogue fragments, and this suggested that these would be effective genetic markers for the assessment of genetic diversity in plants. In addition, BLAST search analysis revealed that these amplified fragments possessed homologies to other genes and proteins in different plant varieties. We conclude that the sequence diversity of P450 gene-analogues in different plant species reflects the diversity of functional regions in the plant genome and is therefore an effective tool in functional genomic studies of plants.

Recessive and dominant genes interfere with the vascular transport of Potato virus A in diploid potatoes.

Resistance to Potato virus A (PVA) was examined in a diploid cross involving Solanum tuberosum subsp. andigena as a resistance source. Hypersensitive resistance (HR) to PVA cosegregated with extreme resistance (ER) to Potato virus Y conferred by the dominant gene Ry(adg) on chromosome XI. Hence, HR to PVA was controlled by a novel, dominant resistance gene closely linked to Ry(adg), or Ry(adg) recognized both viruses but conferred a different type of resistance to each virus. The HR prevented systemic infection with PVA following mechanical inoculation but not following graft inoculation. Another, recessive gene, ra, that may be linked or even allelic with Ry(adg) fully blocked vascular transport of PVA in graft-inoculated plants. Hence, a possibility exists that the genes for the three types of resistance to potyviruses may reside at the same, resistance gene-rich chromosome region syntenic in solanaceous species and might be related. The gene ra acted against all of the three PVA strains tested and, therefore, the avirulence determinants could not be mapped. However, also, PVA strain-specific resistance was found in the progeny. It was overcome by mutations introduced into the viral genome-linked protein and the helper component proteinase and/or the coat protein.

Development of SCAR markers to the PVY resistance gene Ryadg based on a common feature of plant disease resistance genes.

Sequence-characterized amplified regions (SCARs) were developed, based on nucleotide differences within resistance gene-like fragments isolated from a potato plant carrying the Ryadg gene, which confers extreme resistance to potato Y potyvirus (PVY). It originates from Solanum tuberosum subsp. andigena, and a susceptible potato plant. SCARs were tested using 103 potato breeding lines and cultivars with diverse genetic backgrounds derived from Europe, North America, and Japan. Two markers showed high accuracy for detection of the Ryadg gene. The SCAR marker RYSC3 was generated only in genotypes carrying Ryadg. The SCAR marker RYSC4 was detected in all genotypes carrying Ryadg but also in four PVY-susceptible genotypes. Neither marker was detected in genotypes carrying other Ry genes originating from different species than S. tuberosum subsp. andigena. Therefore, these SCAR markers should be powerful tools in marker-assisted selection for Ryadg in potato breeding programs, and should also be useful for cloning of the Ryadg gene.

Examination of the leaf-drop symptom of virus-infected potato using anther culture-derived haploids.

ABSTRACT Necrotic lesions and vein necrosis characteristic of the hypersensitive response (HR) controlled by the dominant resistance gene Ny develop in potato cv. Pito after infection with potato virus Y ordinary strain (PVY masculine) at a low temperature (16/18 degrees C night/day). In contrast, at high temperatures (19/24 degrees C night/day), large coalesced lesions develop in the lower infected leaves, which wither and remain hanging from stems forming the leaf-drop symptom; mosaic symptoms with no necrosis also develop in the top leaves. The genetic basis of the leaf-drop symptom and its dependence on temperature were examined using a novel approach involving 58 haploids (2n = 24) derived from 'Pito' (2n = 48) through anther culture. These haploids and 'Pito' were graft-inoculated with PVY(O) at 19/24 to 25 degrees C (night/day). Necrotic symptoms were expressed in 28 haploids, of which 18 haploids (phenotype class N) developed top necrosis, vein necrosis, or both and necrotic lesions that are characteristic of HR. Ten haploids showed leaf drop similar to 'Pito' (phenotype class LD). Thirty haploids were susceptible and showed only mosaic symptoms (phenotype class S). These data indicated that necrosis was induced by a single dominant gene, Ny, in the simplex condition. However, the three distinct phenotypic classes (N, LD, and S) among the haploids grown under the same environmental conditions showed that another locus (gene) was involved in modifying the HR triggered by Ny. Data suggested that this locus contains a dominant temperature-dependent modifier (Tdm) gene that alters the expression of PVY-induced HR at higher temperatures, resulting in leaf drop.

Overcoming crossing barriers between nontuber-bearing and tuber-bearing Solanum species: towards potato germplasm enhancement with a broad spectrum of solanaceous genetic resources.

The first direct sexual hybrids between diploid nontuber-bearing species and diploid potato breeding lines are reported here. Three nontuberous species of Solanum, S. brevidens, S. etuberosum, and S. fernandezianum, were used for sexual crosses, achieved by a combination of rescue pollinations and embryo rescue. Initial hybrid selection was made using an embryo spot marker, followed by the evaluation of morphological and reproductive traits. Putative hybrids were first tested for resistance to potato leaf roll virus derived from the wild species, and then were tested with molecular markers using species-specific DNA probes. Finally, the tuberization of several 2x hybrids was tested for actual potato germplasm enhancement. These hybrids are unique in terms of their potential to enhance recombination between chromosomes of wild species and those of cultivated potatoes in germplasm utilization, and to exploit the genetic nature of tuber formation. The finding that nontuber-bearing Solanum spp. can be directly crossed with tuber-bearing species also has important implications for the regulatory aspects of the use of genetically modified organisms.

Potato germ plasm enhancement with disomic tetraploid Solanum acaule. II. Assessment of breeding value of tetraploid F1 hybrids between tetrasomic tetraploid S. tuberosum and S. acaule.

The breeding value of tetraploid F1 hybrids between tetrasomic tetraploid S. tuberosum and the disomic tetraploid wild species S. acaule was examined. The F1 hybrids showed a tuber yield and appearance comparable to those of their cultivated parent, indicating a potential as acceptable breeding stocks despite the 50% contribution to their pedigree from wild S. acaule. The cytological behavior of the tetraploid F1 hybrids was examined to determine the probability of recombination for the introgression of S. acaule genes. The majority of the meiotic configurations at metaphase I was bivalents and univalents with mean frequencies of 17.6 and 9.9, respectively. Further, a low frequency of trivalents and quadrivalents was observed. An acceptable low level of meiotic irregularities were observed at the later stages of microsporogenesis, and a reasonable level of pollen stainability was obtained. Therefore, these hybrids could likely be employed for further introgression. From the cytological observations, the following speculations were drawn: (1) some genomic differentiation exists between the S. acaule genomes, (2) at least one of the S. acaule genomes may be homoeologous to the S. tuberosum genomes, (3) intergenomic recombination would likely occur due to the nature of the genomic constitution of the hybrids, and (4) the nature of sesquiploidy of the hybrids may facilitate efficient introgression and establishment of unique aneuploid and euploid recombinant genetic stocks.