How to Make a Superorganism

Insect superorganisms are characterized by a reproductive division of labor (drones, queens, and workers) and a complex division of labor among the non-reproductive individuals, the workers. In the social bees that have attained the highest degrees of sociality, at or approaching superorganism status, males don’t survive mating and are only present as reproductive sperm sequestered in the queen. Queens and workers are anatomically differentiated but derived from the same genome. Differentiation is a consequence of differential feeding of developing larvae by the workers. In the honey bee, worker nurse bees manipulate the developing larvae, forcing them into their reproductive roles. The adult workers self-organize into an ordered society, performing all of the functions necessary for colony survival and reproduction. There are no task masters or forewomen directing the workforce. Instead, every individual makes local decisions about their behavior based on their response thresholds to stimuli in their environment.

Positive selection alone is sufficient for whole genome differentiation at the early stage of speciation process in the fall armyworm

Background: The process of speciation inherently involves the transition from genetic to genomic differentiation. In the absence of a geographic barrier, the whole genome differentiation may occur only when the homogenizing effect of recombination is overcome across the whole genome. The fall armyworm is observed as two sympatric strains with different host-plant preferences across the entire habitat. These two strains exhibit a very low level of genetic differentiation across the whole genome, suggesting that whole genome differentiation occurred at an early stage of speciation. In this study, we aim at identifying critical evolutionary forces responsible for the whole genome differentiation in the fall armyworm. Results: We found that these two strains exhibit a low level of genomic differentiation (Fst = 0.0176), while 91.3% of 10kb windows have genetically differentiated sequences (Fst > 0). We observed that a genomic reduction in migration rate due to combined effects of mild positive selection and genetic linkages to selectively targeted loci are responsible for the whole genome differentiation. Phylogenetic analysis shows that positive selection generates the whole genome differentiation by sub-setting of variants in one strain from the other. Conclusions: From these results, we concluded that positive selection alone is sufficient for whole genome differentiation during the process of speciation. This study demonstrates that the propensity of adaptation alone determines the speciation events, suggesting that adaptive evolution is a single critical driving force for species diversity.

Divergent selection causes whole genome differentiation without physical linkage among the targets in Spodoptera frugiperda (Noctuidae)

ABSTRACTThe 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.