Heterosis effect for larval performance of fall armyworm interstrain hybrids

Spodoptera frugiperda, also known as fall armyworm (FAW), is an invasive crop pest that can feed on a variety of host plants, posing a serious threat to food security. There are two sympatric strains of FAW that are morphologically identical but described with different food preferences: the "rice strain" (SfR) and the "corn strain" (SfC). A few genetic loci exist to identify these two strains. Mitochondrial and Z-chromosome-linked haplotypes are the most used, but the biggest part of the genome displays little polymorphism between strains that could explain their adaptation to different plants. We have previously observed consistent transcription differences between the strains in both laboratory and natural populations. Therefore, we wonder if there are effects from host-strain-associated loci, maternally or paternally inherited, on FAW performance that could explain the divergence between the two FAW strains. To test this hypothesis, we first produced two F₁ hybrid generations (SfR ♀ × SfC ♂, SfC ♀ × SfR ♂). These reciprocal hybrids should be heterozygous for all chromosomes except for the maternally inherited mitochondrial and sexual W chromosomes. To evaluate whether plant preference is determined by these genetic loci, we cultivated the two hybrids and the two parental strains in triplicate on an artificial diet and recorded several phenotypic traits such as weight over time, survival rate, emerging rate, developmental time, and sex ratio. Then, the same performance experiment was carried out on corn plants. Surprisingly, on the artificial diet, the two hybrid genotypes were both more performant than the two parental strains in terms of survival rate, pupal emerging rate, and developmental time, whereas they were intermediate to the inbred parental strains in pupal weight. On the corn plant diet, both hybrid genotypes outperformed the two parental strains in larval weight. Although these asymmetrical results revealed that mitochondrial or sex-linked haplotypes alone cannot explain the performance differences, they suggested a heterosis effect in FAW. A reduction of the female number for the CR genotype and the decreased F₁ offspring reproduction in both hybrids suggested the possibility of Haldane's rule, which might be explained by the dominance model.