57 Surprisingly, we found that species trees estimated from interspecific F ST provided better 58 approximations of mtDNA relationships among the studied species than those estimated using D C ,
59even though F ST was more affected by null alleles. We observed a significantly non-linear second 60 order polynomial relationship between microsatellite and mtDNA distances. We propose that the loss 61 of linearity with increasing mtDNA distance stems from an increasing proportion of homoplastic allele 62 size classes that are identical in state, but not identical by descent. Therefore, despite high cross-63 species amplification success and high polymorphism among the closely related Pachyptila species,
64we caution against the use of microsatellites in phylogenetic inference among distantly related taxa.
Speciation through homoploid hybridization (HHS) is considered extremely rare in animals. This is mainly because the establishment of reproductive isolation as a product of hybridization is uncommon. Additionally, many traits are underpinned by polygeny and/or incomplete dominance, where the hybrid phenotype is an additive blend of parental characteristics. Phenotypically intermediate hybrids are usually at a fitness disadvantage compared with parental species and tend to vanish through backcrossing with parental population(s). It is therefore unknown whether the additive nature of hybrid traits in itself could lead successfully to HHS. Using a multi-marker genetic data set and a meta-analysis of diet and morphology, we investigated a potential case of HHS in the prions (Pachyptila spp.), seabirds distinguished by their bills, prey choice, and timing of breeding. Using approximate Bayesian computation, we show that the medium-billed Salvin’s prion (Pachyptila salvini) could be a hybrid between the narrow-billed Antarctic prion (Pachyptila desolata) and broad-billed prion (Pachyptila vittata). Remarkably, P. salvini’s intermediate bill width has given it a feeding advantage with respect to the other Pachyptila species, allowing it to consume a broader range of prey, potentially increasing its fitness. Available metadata showed that P. salvini is also intermediate in breeding phenology and, with no overlap in breeding times, it is effectively reproductively isolated from either parental species through allochrony. These results provide evidence for a case of HHS in nature, and show for the first time that additivity of divergent parental traits alone can lead directly to increased hybrid fitness and reproductive isolation.
Phytophthora cactorum and P. nicotianae cause leather rot on fruit and crown rot (PhCR) of strawberry plants. Leather rot is not a common disease in Florida; however, up to 50% yield loss has been reported in harvests following intense rainfall events. PhCR is an important disease worldwide and is characterized by a sudden wilting and collapse of plants. Mefenoxam is the most effective and widely used fungicide to control both diseases. P. cactorum and P. nicotianae isolates from leather rot and PhCR have been collected from multiple strawberry fields in Florida since 1997 and the sensitivity of 185 isolates was tested at 0, 0.05, 0.5, 5 and 100 µg/ml. EC50 values of sensitive isolates ranged from 0.05 to 1 µg/ml. Resistance to mefenoxam (EC50 values > 100 µg/ml) was found among P. cactorum isolates collected after 2015 but no resistance was found in P. nicotianae isolates. During the 2015-16, 2016-17, 2017-18, and 2018-19 seasons, resistance was detected on 9, 10, 21, and 23% of the isolates collected, respectively. Mefenoxam-resistant isolates originated from three of the 24 strawberry nurseries monitored. This is the first report of the occurrence of P. cactorum resistance to mefenoxam in Florida, suggesting that alternative control strategies are needed to avoid the increase of mefenoxam-resistant populations of P. cactorum in Florida fields.
Rapid and accurate disease diagnosis is a prerequisite for an effective disease management program in strawberry production. In Florida, Colletotrichum spp., Phytophthora spp, and Macrophomina phaseolina are the primary microorganisms causing strawberry crown rot. Even though the diseases can be caused by different pathogens, symptoms are indistinguishable and equally devastating. To timely inform strawberry growers of diagnostic results for effective deployment of chemical control practices, we developed a multiplex high-resolution melting (HRM) assay to rapidly and accurately detect the above-mentioned pathogens. The multiplex HRM assays using three pre-designed primer pairs showed high specificity for individual species by generating specific melting peaks without cross-reaction between primers or with other common strawberry pathogens. The amplification limit of the assay was 1 pg of Colletotrichum and Phytophthora and 100 pg of M. phaseolina DNA per 10 μl reaction. However, the presence of different melting peaks was observed in mixed DNA samples and was concentration- and target DNA-dependent. A crude DNA extraction protocol was developed to allow high-throughput screening by minimizing the inhibitory effects. Moreover, we applied the HRM assay to 522 plant samples and found high correlations between conventional pathogen isolation and HRM and between singleplex and multiplex assays. Altogether, this multiplex HRM assay is specific, cost-effective, and reliable for the timely detection of strawberry crown rot pathogens.
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