Genetic structure analysis of cultivated and wild chestnut populations reveals gene flow from cultivars to natural stands.

Japanese chestnut (Castanea crenata Sieb. et Zucc.), the only fruit tree species domesticated in Japan, has been cultivated alongside natural stands since prehistorical times. Understanding the genetic diversity of this species and the relationships between cultivated and wild chestnut is important for clarifying its breeding history and determining conservation strategies. We assessed 3 chestnut cultivar populations and 29 wild chestnut populations (618 accessions). Genetic distance analysis revealed that wild populations in the Kyushu region are genetically distant from other populations, whereas other wild and cultivar populations are comparatively similar. Assignment tests suggested that cultivars were relatively similar to populations from central to western Honshu. Bayesian structure analyses showed that wild individuals were roughly classified according to geographical distribution along the Japanese archipelago, except that some wild individuals carried the genetic cluster prevalent in cultivars. Parentage analyses between cultivars and wild individuals identified 26 wild individuals presumed to have a parent-offspring relationship with a cultivar. These results suggested that the genetic structure of some wild individuals in natural stands was influenced by gene flow from cultivars. To conserve wild individuals carrying true "wild" genetic clusters, these individuals should be collected and preserved by ex situ conservation programs.

Construction of genetic linkage map and identification of a novel major locus for resistance to pine wood nematode in Japanese black pine (Pinus thunbergii).

BACKGROUND: Pine wilt disease (PWD), which is caused by the pine wood nematode (PWN) Bursaphelenchus xylophilus, is currently the greatest threat to pine forests in Europe and East Asian countries including Japan. Constructing a detailed linkage map of DNA markers and identifying PWD resistance genes/loci lead to improved resistance in Pinus thunbergii, as well as other Pinus species that are also susceptible to PWD. RESULTS: A total F1 mapping population of 188 individuals derived from a cross between the PWD-resistant P. thunbergii varieties 'Tanabe 54' (resistant rank 2 to PWD) and 'Tosashimizu 63' (resistant rank 4 to PWD) was inoculated with PWN, and was evaluated for disease symptoms. To perform linkage analysis for PWN resistance, a set of three maps was constructed; two parental maps generated using the integrated two-way pseudo-testcross method, and a consensus map with population-type cross-pollination. The linkage map of 'Tanabe 54' consisted of 167 loci, and covered 14 linkage groups (LGs), with a total genetic distance of 1214.6 cM. The linkage map of 'Tosashimizu 63' consisted of 252 loci, and covered 14 LGs, with a total genetic distance of 1422.1 cM. The integrated consensus map comprised 12 LGs with the basic chromosome number of P. thunbergii, and a total genetic distance of 1403.6 cM. Results from quantitative trait loci (QTL) analysis using phenotype data and linkage maps indicated that PWN resistance is controlled by a single dominant allele, which was derived from the 'Tanabe 54' female parent. This major QTL was located on linkage group 3 and was designated PWD1 for PINE WILT DISEASE 1. CONCLUSIONS: The PWD1 locus is a major resistance QTL located on the Pinus consensus LG03 that acts in a dominant manner to confer pine wood nematode resistance. Information from the present study will be useful for P. thunbergii breeding programs to improve resistance to PWD, and also to help identify susceptibility genes in Pinus species.

Consecutive five-year analysis of paternal and maternal gene flow and contributions of gametic heterogeneities to overall genetic composition of dispersed seeds of Pinus densiflora (Pinaceae).

PREMISE OF THE STUDY: Genetic variability in monoecious woody plant populations results from the assemblage of individuals issued from asymmetrical male and female reproductive functions, produced during spatially and temporarily heterogeneous reproductive and dispersal events. Here we investigated the dispersal patterns and levels of genetic diversity and differentiation of both paternal and maternal gametes in a natural population of Pinus densiflora at the multiple-year scale as long as five consecutive years. • METHODS: We analyzed the paternity and maternity for 1576 seeds and 454 candidate adult trees using nuclear DNA polymorphisms of diploid biparental embryos and haploid maternal megagametophytes at eight microsatellite loci. • KEY RESULTS: Despite the low levels of genetic differentiation among gamete groups, a two-way AMOVA analysis showed that the parental origin (paternal vs. maternal gametes), the year of gamete production and their interaction had significant effects on the genetic composition of the seeds. While maternal gamete groups showed a significant FST value across the 5 years, this was not true for their paternal counterparts. Within the population, we found that the relative reproductive contributions of the paternal vs. the maternal parent differed among adult trees, the maternal contributions showing a larger year-to-year fluctuation. • CONCLUSIONS: The overall genetic variability of dispersed seeds appeared to result from two sources of heterogeneity: the difference between paternal and maternal patterns of reproduction and gamete dispersal and year-to-year heterogeneity of reproduction of adult trees, especially in their maternal reproduction.