How does this insect, which is an important model species, feed on poisonous milkweed seeds and use the acquired toxins for its own protection and bright red warning colouration? To explore these phenomena, an international study including SIB Researchers at the Universities of Lausanne and of Geneva sequenced the milkweed bug genome and carried out extensive comparisons with other insects, including sap-sucking aphids and blood-sucking bed bugs, using SIB orthology resources.

[1] Not all bugs are bugs

While many small crawling animals are referred to as “bugs”, the word is in fact a technical term within the insects for the order Hemiptera, including aphids, cicadas, and stink bugs. The vast majority of hemipterans are plant feeders, using their sucking and piercing mouthparts to feed on plant sap, seeds, or other tissues. Meanwhile, some of the most notorious hemipterans, like bed bugs and kissing bugs, instead feed on animal blood. This project formed part of the ongoing i5k initiative, an international consortium seeking to sequence 5’000 genomes of insects and their relatives.

A major boost for insect biodiversity and arthropod evolution studies

“Milkweed bugs [1] are a great species to study,” explains lead investigator Kristen Panfilio from the University of Warwick, UK, and the University of Cologne, Germany. “They have served as a research model for ecology, metabolism, development, and genetics since the mid-twentieth century, in part because they are very easy to keep in the lab. In fact, the strain we sequenced for the genome project is also used in school classrooms, as the bugs are a beautiful red-orange and black colour throughout their life cycles.” Now, the genome data allow researchers to directly link genes with diet and, ultimately, bugs’ wing and body colours. The features of the 926-Mbp Oncopeltus fasciatus genome and comparisons with other insects are reported in the journal Genome Biology. “To date only a few hemipteran species have been sequenced, so the milkweed bug genome represents a major boost for insect biodiversity studies and understanding arthropod evolution,” explains co-author Robert Waterhouse, SIB Group Leader at the Department of Ecology and Evolution, University of Lausanne, Switzerland.

The milkweed bug: a full sensory repertoire and a taste for tough sugars

[2] Putting the milkweed bug genome into context

The team was able to assess the genome assembly quality and compare its content to that of other arthropods using the BUSCO (Benchmarking Universal Single-Copy Ortholog) tool – which provides a quality assessment of genome completeness – and OrthoDB – a comprehensive catalogue of orthologous genes from many species including more than 150 arthropods, which also annotates orthologous groups with functional and evolutionary features. Both resources are developed by the SIB Group of Evgeny Zdobnov, a co-author on the paper.

And how does the milkweed bug compare to its fellow Hemiptera? “To assess the genome and understand how the protein-coding genes identified in the milkweed bug compared to those found in other insects with sequenced genomes, we relied on BUSCO and OrthoDB [2], two made-in-Switzerland orthology tools”, explains Waterhouse.

This new study finds that species with highly specialized liquid diets, like aphids and bed bugs, tend to lose not only certain metabolic enzymes, but even the breadth of their smell and taste receptors. In contrast, the milkweed bug, which preferentially seeks out and feeds on milkweed plants over a broad geographic range, retains a much fuller repertoire of sensory proteins.

At the same time, milkweed bugs and some of their close relatives have newly acquired genes from bacteria, integrating the genes directly into the bug genome. Some of the new genes provide enzymes that help these plant feeders digest the plant tissues’ tough cellulose – a carbohydrate.

The genomic resources made available in this study will allow researchers to further investigate genes important for feeding ecology and linked biological features in milkweed bugs, such as the molecular basis for the red warning pigment as well as bug-specific proteins important for chemical protection and development.

Publication:
Panfilio, K.A., et al. Molecular evolutionary trends and feeding ecology diversification in the Hemiptera, anchored by the milkweed bug genome, 2019, Genome Biology
DOI: https://doi.org/10.1186/s13059-019-1660-0

oncopeltus milkweedbug web

The genome of the milkweed bug (Oncopeltus fasciatus) was sequenced as part of the i5K pilot project. On the right, juveniles and adults congregate on the seed pod of their host plant, the milkweed. Credits: i5k logo by Chiaki Ueda; adult bug photographs by Jena Johnson; seed pod photograph by Deniz Erezyilmaz.