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Pacific beetle-mimic cockroaches may look much like other cockroaches, but they do something no other cockroach does: they give birth to live young. Instead of laying eggs, Diploptera punctata females house their young inside a uterus-like brood sac, and secrete a special “milk” from its walls for nourishment until the offspring emerge as fully-formed cockroaches.
Such viviparity has evolved at least 140 times in animals, including dozens of independent appearances in insects. However, most of what is known about the evolution of this form of reproduction comes from vertebrates. In publishing the first genome sequence for D. punctata, made available February 4 as a bioRxiv preprint, an international team of researchers aims to change that and help broaden the scientific understanding of viviparity.
Project collaborators sequenced and assembled the genome using a combination of PacBio long reads and Illumina short reads. The resulting 3.13 Gb assembly had few gaps and was estimated to be 97.6 percent complete. The researchers also assembled a transcriptome and compared it and the genome to two distantly related viviparous insects, aphids (Acyrthosiphon pisum and Rhopalosiphum maidis) and tsetse flies (Glossina species). This revealed convergence in gene family expansions and contractions and rapid evolution in genes associated with development of the heart, tracheal system, and reproductive organs, as well as expression changes to genes involved in chitin metabolism and a reduction of immunity during pregnancy. Additionally, the researchers uncovered evidence for selection on genes involved in cell-cell adhesion, membrane trafficking, oxidative metabolism, and embryonic development.
The study suggests that all three species independently evolved urogenital remodeling, alterations to maternal control of embryo development, and tweaks to the development of the tracheal system and heart, the authors write. “Our findings suggest the essential role of those pathways for the development of [a] placenta-like structure enabling embryo development and nutrition.” Intriguingly, they continue, many of the genomic clues in insects echo what is known about viviparous transitions in vertebrates.
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Stick insects (Timema species)
Stick insects have evolved parthenogenesis, when embryos spontaneously develop from unfertilized egg cells, multiple times despite the potential genomic pitfalls of asexual reproduction, including the loss of genetic diversity. To see if these animals actually suffered such consequences of their reproductive strategy, researchers sequenced the genomes of multiple individuals in populations of 10 different Timema species, half of which reproduce sexually and half of which reproduce parthenogenically. As expected, “parthenogenesis results in an extreme reduction of heterozygosity and often leads to genetically uniform populations,” the authors write in a February 23 paper in Science Advances. Still, they note, parthenogenesis is “an unusually successful strategy” in the genus, so the genetic price of forgoing sex must be offset by some benefit. Given the animals’ fire-prone habitat and lack of wings, the researchers posit that it may be difficult enough to find mates that it’s worth the costs of reproducing solo.
Silveira strain (Coccidioides posadasii)
Every year, tens of thousands of people in the Americas are diagnosed with the fungal disease coccidioidomycosis, or Valley fever, with many additional cases likely overlooked because of limited testing. Vaccine and treatment development programs as well as the antigens for existing tests largely rely on a single strain of fungus: Coccidioides posadasii Silveira strain. While a draft genome for this strain was produced in 2009, it was compiled from Sanger sequencing (the DNA sequencing method that predated next-gen approaches) and was woefully incomplete. Now, thanks to long-read PacBio sequencing, researchers have constructed a high-quality, chromosome-level genome to aid in efforts to understand and treat the disease. The new genome was published February 7 in G3 Genes|Genomes|Genetics.
Genome Spotlight is a monthly column for The Scientist’s Genetics & Genomics newsletter that highlights recently published genome sequences and the mysteries of life they may reveal.