Patterns of genomic and allochronic strain divergence in the fall armyworm, Spodoptera frugiperda (JE Smith)

Speciation is the process through which reproductive isolation develops between distinct populations. Because this process takes time, speciation studies often necessarily examine populations within a species that are at various stages of divergence. The fall armyworm, Spodoptera frugiperda (J.E. Smith), is comprised of two strains (R = Rice & C = Corn) that serve as a novel system to explore population divergence in sympatry. Here, we use ddRADSeq data to show that fall armyworm strains in the field are largely genetically distinct, but some interstrain hybridization occurs. Although we detected F1 hybrids of both R- and C-strain maternal origin, only hybrids with R-strain mtDNA were found to contribute to subsequent generations, possibly indicating a unidirectional barrier to gene flow. Although these strains have been previously defined as host plant-associated, we recovered an equal proportion of R- and C-strain moths in fields dominated by C-strain host plants. As an alternative to host-associated divergence, we tested the hypothesis that differences in nightly activity patterns could account for reproductive isolation by genotyping temporally collected moths. Our data indicates that strains exhibit a significant shift in the timing of their nightly activities in the field. This divergence in phenology creates a prezygotic reproductive barrier that likely maintains the genetic isolation between strains. Thus, we conclude that it may be ecologically inaccurate to refer to the C- and R- strain as host-associated and they should more appropriately be considered allochronic strains.

Automated summary

Speciation refers to the process of reproductive isolation developing between distinct populations. In the study of fall armyworm, it was discovered that there are significant genetic differences between strains in the field, with some interstrain hybridization occurring. The hybrids with specific strain mitochondrial DNA were found to contribute to subsequent generations, suggesting a unidirectional barrier to gene flow. Additionally, the strains exhibited significant differences in their nightly activity patterns, which may create a prezygotic reproductive barrier maintaining genetic isolation between them. Consequently, referring to the strains as host-associated may be ecologically inaccurate, and they should be considered allochronic strains.