Genomics illuminates mysteries of monarch flight, color

The mystery of how ephemeral monarch butterflies migrate thousands of miles each year has yielded at least some of its secrets to detailed genomic study. A collaborative research team reports in Nature that its genome-wide single-nucleotide polymorphism analysis of 101 geographically dispersed specimens revealed the underlying pattern of species dispersal from the monarch’s ancestral home in North America. The research team compared the genomes of migratory and nonmigratory monarchs to identify several genetic variations associated with long-distance migration. These genes, which affect flight muscle, result in more efficient energy consumption and lower resting metabolic rates. The pattern of genetic variation also revealed that, after dispersing to Europe, South America, and Hawaii, among other locations, the monarchs reverted to a nonmigratory form. In the process, a single collagen gene changed in the same way in all dispersed nonmigrating populations.

Additional comparison between the genomes of five white monarchs and seven orange monarchs from Hawaii identified variation in a single gene—the myosin gene DPOGS206617—as being responsible for wing pigmentation. The gene previously had no known role in insect pigmentation but was known to affect mouse-coat coloration. This finding was a surprise to investigators, who had expected the gene variation to disrupt pigment formation. Instead, the nivosus mutation seems to impair pigment transport. While genomics can inform us about the monarch’s evolutionary past, it will take intensive conservation to preserve the monarch’s annual migration; 2013 saw the lowest numbers of migrants in recorded history, due to habitat loss and the decline of its primary food source, milkweed. —Karyn Hede, News Editor

Some insects use rare structure to separate chromosomes

Given the essential role of centromeres, which provide the critical spindle-fiber attachment point to chromosomes during cell division, it’s not surprising that a molecular “hook,” a histone protein called CenH3, is conserved from fungi to animals. It now appears, however, that isn’t the case in several orders of insects comprising 16% of all known eukaryotic species. In a comprehensive survey of available insect genomes and transcriptomes, a research team from Fred Hutchinson Cancer Research Center and the University of Washington showed that several lineages of insects independently evolved a decentralized mechanism of spindle-fiber attachment points that don’t require the CenH3 attachment hook previously thought to be indispensable. Instead, many butterflies, dragonflies, damselflies, and earwigs, as well as many true bugs, have spindle-fiber attachment points along the entire length of chromosomes. These holocentric chromosomes, the research team hypothesizes, do not require CenH3, and therefore the gene was lost in many insect species during evolution. By contrast, other holocentric species, such as the nematode Caenorhabditis elegans, retain CenH3. The research report, published in September 2014 in eLife, challenges long-established assumptions about the very foundation of cell division and may provide new avenues to understanding the fundamental requirements underlying pathologies related to chromosomal imbalances, including aneuploidy and cancer. —Karyn Hede, News Editor

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