No abstract

The genus Populus consists of dioecious woody species with largely unknown genetic mechanisms for gender determination. We have discovered genetic and genomic features in the peritelomeric region of chromosome XIX that suggest this region of the Populus genome is in the process of developing characteristics of a sex chromosome. We have identified a gender-associated locus that consistently maps to this region. Furthermore, comparison of genetic maps across multiple Populus families reveals consistently distorted segregation within this region. We have intensively characterized this region using an F 1 interspecific cross involving the female genotype that was used for genome sequencing. This region shows suppressed recombination and high divergence between the alternate haplotypes, as revealed by dense map-based genome assembly using microsatellite markers. The suppressed recombination, distorted segregation, and haplotype divergence were observed only for the maternal parent in this cross. Furthermore, the progeny of this cross showed a strongly male-biased sex ratio, in agreement with Haldane's rule that postulates that the heterogametic sex is more likely to be absent, rare, or sterile in interspecific crosses. Together, these results support the role of chromosome XIX in sex determination and suggest that sex determination in Populus occurs through a ZW system in which the female is the heterogametic gender.

The capacity to root from cuttings is a key factor for the mass deployment of superior genotypes in clonal forestry. We studied the genetic basis of rooting capacity by mapping quantitative trait loci (QTLs) that control growth rate and form of root traits in a full-sib family of 93 hybrids derived from an interspecific cross between two Populus species, P. deltoides and P. euramericana. The hybrid family was typed for different marker systems (including SSRs, AFLPs, RAPDs, ISSRs, and SNPs), leading to the construction of two linkage maps based on the female P. deltoides (D map) and male P. euramericana (E map) with a pseudotestcross mapping strategy. The two maps were scanned by functional mapping to detect QTLs that control early growth trajectories of two rooting traits, maximal single-root length and the total number of roots per cutting, measured at five time points in water culture. Of the six QTLs detected for these two growth traits, only one is segregating in P. deltoides with poor rooting capacity, while the other five are segregating in P. euramericana showing good rooting capacity. Tests with functional mapping suggest different developmental patterns of the genetic effects of these root QTLs in time course. Five QTLs were detected to change their effects on root growth trajectories with time, whereas one detected to affect root growth consistently in time course. Knowledge about the genetic and developmental control mechanisms of root QTLs will have important implications for the genetic improvement of vegetative propagation traits in Populus.

We report molecular genetic linkage maps for an interspecific hybrid population of Populus, a model system in forest-tree biology. The hybrids were produced by crosses between P. deltoides (mother) and P. euramericana (father), which is a natural hybrid of P. deltoides (grandmother) and P. nigra (grandfather). Linkage analysis from 93 of the 450 backcross progeny grown in the field for 15 years was performed using random amplified polymorphic DNAs (RAPDs), amplified fragment length polymorphisms (AFLPs), and inter-simple sequence repeats (ISSRs). Of a total of 839 polymorphic markers identified, 560 (67%) were testcross markers heterozygous in one parent but null in the other (segregating 1:1), 206 (25%) were intercross dominant markers heterozygous in both parents (segregating 3:1), and the remaining 73 (9%) were 19 non-parental RAPD markers (segregating 1:1) and 54 codominant AFLP markers (segregating 1:1:1:1). A mixed set of the testcross markers, non-parental RAPD markers, and codominant AFLP markers was used to construct two linkage maps, one based on the P. deltoides (D) genome and the other based on P. euramericana (E). The two maps showed nearly complete coverage of the genome, spanning 3801 and 3452 cM, respectively. The availability of non-parental RAPD and codominant AFLP markers as orthologous genes allowed for a direct comparison of the rate of meiotic recombination between the two different parental species. Generally, the rate of meiotic recombination was greater for males than females in our interspecific poplar hybrids. The confounded effect of sexes and species causes the mean recombination distance of orthologous markers to be 11% longer for the father (P. euramericana; interspecific hybrid) than for the mother (P. deltoides; pure species). The linkage maps constructed and the interspecific poplar hybrid population in which clonal replicates for individual genotypes are available present a comprehensive foundation for future genomic studies and quantitative trait locus (QTL) identification.

Loci associated with variation in maize responses to two microbe-associated molecular patterns (MAMPs) were identified. MAMP responses were correlated. No relationship between MAMP responses and quantitative disease resistance was identified. Microbe-associated molecular patterns (MAMPs) are highly conserved molecules commonly found in microbes which can be recognized by plant pattern recognition receptors. Recognition triggers a suite of responses including production of reactive oxygen species (ROS) and nitric oxide (NO) and expression changes of defense-related genes. In this study, we used two well-studied MAMPs (flg22 and chitooctaose) to challenge different maize lines to determine whether there was variation in the level of responses to these MAMPs, to dissect the genetic basis underlying that variation and to understand the relationship between MAMP response and quantitative disease resistance (QDR). Naturally occurring quantitative variation in ROS, NO production, and defense genes expression levels triggered by MAMPs was observed. A major quantitative traits locus (QTL) associated with variation in the ROS production response to both flg22 and chitooctaose was identified on chromosome 2 in a recombinant inbred line (RIL) population derived from the maize inbred lines B73 and CML228. Minor QTL associated with variation in the flg22 ROS response was identified on chromosomes 1 and 4. Comparison of these results with data previously obtained for variation in QDR and the defense response in the same RIL population did not provide any evidence for a common genetic basis controlling variation in these traits.