Resistance against strongylid nematodes in two high prevalence Equine Recurrent Airway Obstruction families has a genetic basis

A recent study showed increased resistance against strongylid nematodes in offspring of a stallion affected by recurrent airway obstruction (RAG) compared with unrelated pasture mates. Resistance against strongylid nematodes was associated with RAG affection. Hypothesis: Resistance against strongylid nematodes has a genetic basis. The genetic variants influencing strongylid resistance also influence RAG susceptibility. Faecal samples from the half-sibling offspring of two RAG-affected Warmblood stallions 98 offspring from the first family (family 1) and 79 from the second family (family 2) were analysed using a combined sedimentation-flotation method. The phenotype was defined as a binary trait - either positive or negative for egg shedding. The influence of non-genetic factors on egg shedding was analysed using SAS, the mode of inheritance was investigated using PAP and iBay, and the association between shedding of strongyle eggs and RAG was estimated by odds ratios. Previously established genotypes for 315 microsatellite markers were used for QTL analyses using GRID QTL. The inheritance of "strongylid egg shedding" is influenced by major genes on ECA15 and ECA20. Shedding of strongylid eggs is associated with RAG in family 1 but not in family 2. Conclusions: The status of "shedding of strongyle eggs" has a genetic background. The results were inconclusive as to whether "egg shedding" and RAG share common genetic components. Our results suggest that it may be possible to select for resistance against strongylid nematodes.

Natural Variation in Maize Aphid Resistance Is Associated with 2,4-Dihydroxy-7-Methoxy-1,4-Benzoxazin-3-One Glucoside Methyltransferase Activity

Plants differ greatly in their susceptibility to insect herbivory, suggesting both local adaptation and resistance tradeoffs. We used maize (Zea mays) recombinant inbred lines to map a quantitative trait locus (QTL) for the maize leaf aphid (Rhopalosiphum maidis) susceptibility to maize Chromosome 1. Phytochemical analysis revealed that the same locus was also associated with high levels of 2-hydroxy-4,7-dimethoxy-1,4-benzoxazin-3-one glucoside (HDMBOA-Glc) and low levels of 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one glucoside (DIMBOA-Glc). In vitro enzyme assays with candidate genes from the region of the QTL identified three O-methyltransferases (Bx10a-c) that convert DIMBOA-Glc to HDMBOA-Glc. Variation in HDMBOA-Glc production was attributed to a natural CACTA family transposon insertion that inactivates Bx10c in maize lines with low HDMBOA-Glc accumulation. When tested with a population of 26 diverse maize inbred lines, R. maidis produced more progeny on those with high HDMBOA-Glc and low DIMBOA-Glc. Although HDMBOA-Glc was more toxic to R. maidis than DIMBOA-Glc in vitro, BX10c activity and the resulting decline of DIMBOA-Glc upon methylation to HDMBOA-Glc were associated with reduced callose deposition as an aphid defense response in vivo. Thus, a natural transposon insertion appears to mediate an ecologically relevant trade-off between the direct toxicity and defense-inducing properties of maize benzoxazinoids.