NEW YORK (GenomeWeb News) – In Cell this week, researchers from the University of Massachusetts Medical School and the California sequencing and informatics firm Genome Project Solutions describe how they sequenced a draft version of the monarch butterfly genome and scoured it for insights into long distance migration.
The monarch, Danaus plexippus, is the first butterfly species to have its genome sequenced. By comparing this 273 million base draft genome with sequences for other species — including the silkmoth, another insect in the same order — the team started to narrow in on genes and microRNAs that seem to help the monarch navigate during its seasonal migrations.
"The monarch genome enhances our ability to better understand the genetic and molecular basis of long distance migration," University of Massachusetts neurobiology researcher Steven Reppert, the study's senior author, and co-authors wrote.
"With the genome in place, deciphering the molecular machinery underlying the monarch's navigational prowess can now begin in earnest," Max Planck Institute for Chemical Ecology researchers Marcus Stensmyr and Bill Hansson, who were not involved in the study, noted in their preview of the new paper in the same issue of Cell.
Monarch butterflies put reproduction on hold each fall and migrate thousands of miles from North America to wintering spots in central Mexico, Reppert and his colleagues explained. In spring, the butterflies shake off this reproductive diapause, reproduce, and fly north again to lay fertilized eggs on milkweed plants in the southern US. After that, spring and summer monarchs return to their summer range sites.
The migrations appear to be a consequence of genetics, since the butterflies making the trip south each fall belong to different generations than those that took the trip before.
"Migratory monarchs are at least two generations removed from those that made the journey the previous fall," Reppert said in a statement. "They have never been to the over-wintering sites before, and have no relatives to follow on their way."
"There must be a genetic program underlying the butterflies' migratory behavior," he added. "We want to know what that program is, and how it works."
To start investigating that, he and his colleagues used Illumina and Roche 454 platforms to sequence genomic DNA from wild, female, migratory monarch butterflies. Combining reads generated with both platforms provided an average of nearly 75 times coverage of the monarch genomes, which researchers assembled de novo.
Meanwhile, Illumina sequencing of RNA from monarch tissues at different stages of development provided 228 times coverage of the butterfly transcriptome — information that was used to help annotate the genome and to explore RNA differences between summer monarchs and their migratory counterparts.
The team identified 16,866 predicted protein-coding genes in the genome, along with 116 miRNAs and 431 transfer RNAs.
Next, researchers began comparing the genome with sequences from other species, including the silkmoth, Bombyx mori, which was sequenced by members of the International Silkworm Genome Consortium in 2008. They also did comparisons involving a dozen other insect species and two mammals.
Along with insights into similarities and differences between the monarch and silkmoth genomes, the team found evidence that the moth and butterfly-containing insect order Lepidoptera has evolved more quickly than other insect orders characterized so far.
In particular, initial analyses of the monarch genome offered insights into the genes that help the butterfly take visual cues from the sun to get directional information, process this information in the brain, and adjust its sun compass based on input from circadian clock components.
The researchers also found some hormone synthesis, chemical and odor sensing, and defense-related genes that appear to be important to monarch butterfly biology and migration, along with a set of miRNAs that is differentially expressed in butterflies at summer sites compared to those migrating south in the fall.
Given the migratory abilities of the monarch, those involved in the study argue that information in its genome and proteome are a "treasure trove" for not only understanding monarch migration and population patterns, but also for investigating migration in other species.
"Dissecting the genetic basis of long-distance migration in the monarch may help us understand these mechanisms not only in monarchs but more generally in other migrants, including migratory birds and sea turtles," Reppert said.