Biology subjects

NAM, K.

Publications and source records attributed to NAM, K..

3 recordsLinked to original sources

bioRxiv2022

Host-plant adaptation as a driver of incipient speciation in the fall armyworm (Spodoptera frugiperda)

BackgroundDivergent selection on host-plants is one of the main evolutionary forces driving ecological speciation in phytophagous insects. The ecological speciation might be challenging in the presence of gene flow and assortative mating because the direction of divergence is not necessarily the same between ecological selection (through host-plant adaptation) and assortative mating. The fall armyworm (FAW), a major lepidopteran pest species, is composed of two sympatric strains, corn and rice strains, named after two of their preferred host-plants. These two strains have been hypothesized to undergo incipient speciation, based on (i) several lines of evidence encompassing both pre- and post-zygotic reproductive isolation, and (ii) the presence of a substantial level of genetic differentiation. Even though the status of these two strains has been established a long time ago, it is still yet to be found whether these two strains indeed exhibit a marked level of genetic differentiation from a large number of genomic loci. Here, we analyzed whole genome sequences from 56 FAW individuals either collected from pasture grasses (a part of the favored host range of the rice strain) or corn to assess the role of host-plant adaptation in incipient speciation. ResultsPrincipal component analysis of whole genome data shows that the pattern of divergence in the fall armyworm is predominantly explained by the genetic differentiation associated with host-plants. The level of genetic differentiation between corn and rice strains is particularly marked in the Z chromosome. We identified one autosomal locus and two Z chromosome loci targeted by selective sweeps specific to rice strain and corn strain, respectively. The autosomal locus has both increased DXY and FST while the Z chromosome loci had decreased DXY and increased FST. ConclusionThese results show that the FAW population structure is dominated by the genetic differentiation between corn and rice strains. This differentiation involves divergent selection targeting at least three loci, which include a locus potentially causing reproductive isolation. Taken together, these results suggest the evolutionary scenario that host-plant speciation is a driver of incipient speciation in the fall armyworm.

evolutionary biology↗

bioRxiv2019

Adaptation by copy number variation increases insecticide resistance in fall armyworms

Insecticide resistance is a major main challenge in pest control, and understanding its genetic basis is a key topic in agricultural ecology. Detoxification genes are well-known genetic elements that play a key role in adaptation to xenobiotics. The adaptive evolution of detoxification genes by copy number variations has been interpreted as a cause of insecticide resistance. However, the same pattern can be generated by the adaptation to host-plant defense toxins as well. In this study, we tested in fall armyworms (Lepidoptera Spodoptera frugiperda) if adaptation by copy number variation is the cause of the increased level of insecticide resistance from two geographic populations with different levels of resistance and two strains with different host plants. Following the generation of an assembly with chromosome-sized scaffolds (N50 = 13.2Mb), we observed that these two populations show a significant allelic differentiation of copy number variations, which is not observed between strains. In particular, a locus with almost complete allelic differentiation (Fst > 0.8) includes a cluster of P450 genes, which are well-known key players in detoxification. Detoxification genes are overrepresented in the genes with copy number variations, and the observed copy number variation appears to have beneficial effects in general. From this result, we concluded that copy number variation of detoxification genes in fall armyworms plays a key role in the insecticide resistance but not in the adaptation to host-plants, suggesting that the evolution of insecticide resistance may occur independently from host-plant adaptation.

evolutionary biology↗

bioRxiv2018

Genomic differentiation is initiated without physical linkage among targets of divergent selection in Fall armyworms

The process of speciation involves whole genome differentiation by overcoming gene flow between diverging populations. We have ample knowledge which evolutionary forces may cause genomic differentiation, and several speciation models have been proposed to explain the transition from genetic to genomic differentiation. However, it is still unclear what are critical conditions enabling genomic differentiation in nature. The Fall armyworm, Spodoptera frugiperda, is observed as two sympatric strains that have different host-plant ranges, suggesting the possibility of ecological divergent selection. In our previous study, we observed that these two strains show genetic differentiation across the whole genome with an unprecedentedly low extent, suggesting the possibility that whole genome sequences started to be differentiated between the strains. In this study, we analyzed whole genome sequences from these two strains from Mississippi to identify critical evolutionary factors for genomic differentiation. The genomic Fst is low (0.017) while 91.3% of 10kb windows have Fst greater than 0, suggesting genome-wide differentiation with a low extent. We identified nearly 400 outliers of genetic differentiation between strains, and found that physical linkage among these outliers is not a primary cause of genomic differentiation. Fst is not significantly correlated with gene density, a proxy for the strength of selection, suggesting that a genomic reduction in migration rate dominates the extent of local genetic differentiation. Our analyses reveal that divergent selection alone is sufficient to generate genomic differentiation, and any following diversifying factors may increase the level of genetic differentiation between diverging strains in the process of speciation.

evolutionary biology↗