The Alba phenotypes of representative Colias species, the evolutionary relationships between the major Colias lineages in light of their Alba phenotypes and their regional distribution, and evidence of historical introgression. (A) Representative Colias species from both sides of the Atlantic illustrating the female-restricted Alba phenotype along with a table of life-history differences between the female morphs. (B) A densitree diagram of trees at the chromosome level (one tree per chromosome) generated using gene trees based on a single exon per single copy gene (on average 129 genes per chromosome; n = 4011 BUSCO genes). (C) The wing color of each specimen is indicated with colored boxes at the branch tips (blue = alba, orange = colored, gray = NA). (D) ASTRAL species tree generated using the longest exon per BUSCO gene, with branches color-coded by the regional distribution of their sample (blue = North America, orange = Holarctic, green = Eurasia and Africa, purple = South America). (E) Time-calibrated SNAPP tree generated with a subset of taxa and 1314 SNPs, with millions of years on the x-axis. Blue bars at nodes represent the 95% highest posterior distribution of node age, with nodes having posterior support from Science Advances (2023). DOI: 10.1126/sciadv.abq3713

Have you ever marveled at the great diversity of life on our planet, from tiny creatures that live only a few hours to majestic beings that can survive for centuries? These differences in lifespan, size, and reproductive age are known as life history strategies and have evolved over time as organisms adapted to their environment.

Evolutionary biologists have long been interested in understanding the factors that contribute to the evolution and maintenance of multiple alternative life history strategies (ALHS) within species that lead to adaptation and new traits. A new study published in scientific advances has not only shown that an ALHS in Colias butterflies has an ancient origin, but also determined the mechanisms that contribute to its persistence over millions of generations.

Colias butterflies are a charismatic group of butterflies found on almost every continent and are distinguished by their bright orange or yellow wings in both males and females. However, in about a third of the 90 Colias species, a proportion of females have white-colored wings called alba morphs instead. This color change appears to be a compromise in how females invest energy reserves accumulated in the caterpillar stage.

During transformation into butterflies, females either spend these reserves to form orange wings and become very attractive to males, or they produce no color (alba) and use these resources to have more offspring. Therefore, Alba is not just a simple color variant, but a visual representation of an ALHS unique to female Colias butterflies.

Led by researchers from Stockholm University, the international team used various genomic analyzes to identify the genetic basis of female-restricted Alba ALHS and then place this discovery in an evolutionary framework. They found that Alba once evolved near the last common ancestor of the genus, more than 1.2 million generations ago.

Their results further suggest that the alba and orange alleles were maintained between species within the genus via gene flow between hybridizing species (introgression) and through balanced selection within species. The genetic basis of Alba appears to be a regulatory region in DNA, a hypothesis the researchers tested using CRISPR/Cas9 mutagenesis that confirmed the Alba allele as a modular enhancer for induction of Alba ALHS.

These results are significant as they provide a better understanding of how life history traits evolve, suggesting that some strategies used by different species may in fact share a common genetic basis.

“By examining the origins and evolutionary dynamics at work on an ALHS, we hope to advance our understanding of how novel traits and life histories evolve, and ultimately help us understand the conditions that govern the diversity of generated variations in life history that we observe today,” said Kalle Tunstrom, Ph.D. Student at the Department of Zoology, Stockholm University and lead author of the study.

More information:
Kalle Tunström et al, Evidence for a single, ancient origin of a species-wide alternative life history strategy, scientific advances (2023). DOI: 10.1126/sciadv.abq3713

Journal Information:
scientific advances


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