Positive Selection and Gene Expression Analyses from Salivary Glands Reveal Discrete Adaptations within the Ecologically Diverse Bat Family Phyllostomidae

Immunological features of bats: resistance and tolerance to emerging viruses

Bats are among the most diverse mammalian species, representing over 20% of mammalian diversity. The past two decades have witnessed a disproportionate spillover of viruses from bats to humans compared with other mammalian hosts, attributed to the viral richness within bats, their phylogenetic likeness to humans, and increased human contact with wildlife. Unique evolutionary adaptations in bat genomes, particularly in antiviral protection and immune tolerance genes, enable bats to serve as reservoirs for pandemic-inducing viruses. Here, we discuss current limitations and advances made in understanding the role of bats as drivers of pandemic zoonoses. We also discuss novel technologies that have revealed spatial, dynamic, and physiological factors driving virus and host coevolution.

The Fast and the Furriest: Investigating the Rate of Selection on Mammalian Toxins

The evolution of venom and the selection pressures that act on toxins have been increasingly researched within toxinology in the last two decades, in part due to the exceptionally high rates of diversifying selection observed in animal toxins. In 2015, Sungar and Moran proposed the 'two-speed' model of toxin evolution linking evolutionary age of a group to the rates of selection acting on toxins but due to a lack of data, mammals were not included as less than 30 species of venomous mammal have been recorded, represented by elusive species which produce small amounts of venom. Due to advances in genomics and transcriptomics, the availability of toxin sequences from venomous mammals has been increasing. Using branch- and site-specific selection models, we present the rates of both episodic and pervasive selection acting upon venomous mammal toxins as a group for the first time. We identified seven toxin groups present within venomous mammals, representing Chiroptera, Eulipotyphla and Monotremata: KLK1, Plasminogen Activator, Desmallipins, PACAP, CRiSP, Kunitz Domain One and Kunitz Domain Two. All but one group (KLK1) was identified by our results to be evolving under both episodic and pervasive diversifying selection with four toxin groups having sites that were implicated in the fitness of the animal by TreeSAAP (Selection on Amino Acid Properties). Our results suggest that venomous mammal ecology, behaviour or genomic evolution are the main drivers of selection, although evolutionary age may still be a factor. Our conclusion from these results indicates that mammalian toxins are following the two-speed model of selection, evolving predominately under diversifying selection, fitting in with other younger venomous taxa like snakes and cone snails-with high amounts of accumulating mutations, leading to more novel adaptions in their toxins.

Evolutionary history of the vertebrate Piwi gene family

PIWIs are regulatory proteins that belong to the Argonaute family. Piwis are primarily expressed in gonads and protect the germline against the mobilization and propagation of transposable elements (TEs) through transcriptional gene silencing. Vertebrate genomes encode up to four Piwi genes: Piwil1, Piwil2, Piwil3 and Piwil4, but their duplication history is unresolved. We leveraged phylogenetics, synteny and expression analyses to address this void. Our phylogenetic analysis suggests Piwil1 and Piwil2 were retained in all vertebrate members. Piwil4 was the result of Piwil1 duplication in the ancestor of gnathostomes, but was independently lost in ray-finned fishes and birds. Further, Piwil3 was derived from a tandem Piwil1 duplication in the common ancestor of marsupial and placental mammals, but was secondarily lost in Atlantogenata (Xenarthra and Afrotheria) and some rodents. The evolutionary rate of Piwil3 is considerably faster than any Piwi among all lineages, but an explanation is lacking. Our expression analyses suggest Piwi expression has mostly been constrained to gonads throughout vertebrate evolution. Vertebrate evolution is marked by two early rounds of whole genome duplication and many multigene families are linked to these events. However, our analyses suggest Piwi expansion was independent of whole genome duplications.

Nectar-feeding bats and birds show parallel molecular adaptations in sugar metabolism enzymes

In most vertebrates, the demand for glucose as the primary substrate for cellular respiration is met by the breakdown of complex carbohydrates, or energy is obtained by protein and lipid catabolism. In contrast, a few bat and bird species have convergently evolved to subsist on nectar, a sugar-rich mixture of glucose, fructose, and sucrose.^1-4 How these nectar-feeders have adapted to cope with life-long high sugar intake while avoiding the onset of metabolic syndrome and diabetes^5-7 is not understood. We analyzed gene sequences obtained from 127 taxa, including 22 nectar-feeding bat and bird genera that collectively encompass four independent origins of nectarivory. We show these divergent taxa have undergone pervasive molecular adaptation in sugar catabolism pathways, including parallel selection in key glycolytic and fructolytic enzymes. We also uncover convergent amino acid substitutions in the otherwise evolutionarily conserved aldolase B (ALDOB), which catalyzes rate-limiting steps in fructolysis and glycolysis, and the mitochondrial gatekeeper pyruvate dehydrogenase (PDH), which links glycolysis and the tricarboxylic acid cycle. Metabolomic profile and enzyme functional assays are consistent with increased respiratory flux in nectar-feeding bats and help explain how these taxa can both sustain hovering flight and efficiently clear simple sugars. Taken together, our results indicate that nectar-feeding bats and birds have undergone metabolic adaptations that have enabled them to exploit a unique energy-rich dietary niche among vertebrates.