Fatty acid metabolism decreased while sexual selection increased in brown rats spreading south

For mammals that originate in the cold north, adapting to warmer environments is crucial for southwards invasion. The brown rat (Rattus norvegicus) originated in Northeast China and has become a global pest. R. n. humiliatus (RNH) spread from the northeast, where R. n. caraco (RNC) lives, to North China and diverged to form a subspecies. Genomic analyses revealed that subspecies differentiation was promoted by temperature but impeded by gene flow and that genes related to fatty acid metabolism were under the strongest selection. Transcriptome analyses revealed downregulated hepatic genes related to fatty acid metabolism and upregulated those related to pheromones in RNH vs. RNC. Similar patterns were observed in relation to cold/warm acclimation. RNH preferred mates with stronger pheromone signals intra-populationally and more genetic divergence inter-populationally. We concluded that RNH experienced reduced fat utilization and increased pheromone-mediated sexual selection during its invasion from the cold north to warm south.

Genotypic detection of barriers to rat dispersal: Rattus rattus behind a peninsula predator-proof fence

Clear delimitation of management units is essential for effective management of invasive species. Analysis of population genetic structure of target species can improve identification and interpretation of natural and artificial barriers to dispersal. In Aotearoa New Zealand where the introduced ship rat (Rattus rattus) is a major threat to native biodiversity, effective suppression of pest numbers requires removal and limitation of reinvasion from outside the managed population. We contrasted population genetic structure in rat populations over a wide scale without known barriers, with structure over a fine scale with potential barriers to dispersal. MtDNA D-loop sequences and microsatellite genotypes resolved little genetic structure in southern North Island population samples of ship rat 100 km apart. In contrast, samples from major islands differed significantly for both mtDNA and nuclear markers. We also compared ship rats collected within a small peninsula reserve bounded by sea, suburbs and, more recently, a predator fence with rats in the surrounding forest. Here, mtDNA did not differ but genotypes from 14 nuclear loci were sufficient to distinguish the fenced population. This suggests that natural (sea) and artificial barriers (town, fence) are effectively limiting gene flow among ship rat populations over the short distance (~ 500 m) between the peninsula reserve and surrounding forest. The effectiveness of the fence alone is not clear given it is a recent feature and no historical samples exist; resampling population genetic diversity over time will improve understanding. Nonetheless, the current genetic isolation of the fenced rat population suggests that rat eradication is a sensible management option given that reinvasion appears to be limited and could probably be managed with a biosecurity programme.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10530-023-03004-8.

Abundance and Dynamics of Small Mammals in New Zealand: Sequential Invasions into an Island Ecosystem Like No Other

New Zealand had no people or four-footed mammals of any size until it was colonised by Polynesian voyagers and Pacific rats in c. 1280 AD. Between 1769 and 1920 AD, Europeans brought three more species of commensal rats and mice, and three predatory mustelids, plus rabbits, house cats hedgehogs and Australian brushtail possums. All have in turn invaded the whole country and many offshore islands in huge abundance, at least initially. Three species are now reduced to remnant populations, but the other eight remain widely distributed. They comprise an artificial but interacting and fully functional bottom-up predator-prey system, responding at all levels to interspecific competition, habitat quality and periodic resource pulsing.

Comparative phylogeography of two commensal rat species (Rattus tanezumi and Rattus norvegicus) in China: Insights from mitochondrial DNA, microsatellite, and 2b-RAD data

Rattus norvegicus and Rattus tanezumi are dominant species of Chinese house rats, but the colonization and demographic history of two species in China have not been thoroughly explored. Phylogenetic analyses with mitochondrial DNA including 486 individuals from 31 localities revealed that R. norvegicus is widely distributed in China, R. tanezumi is mainly distributed in southern China with currently invading northward; northeast China was the natal region of R. norvegicus, while the spread of R. tanezumi in China most likely started from the southeast coast. A total of 123 individuals from 18 localities were subjected to 2b-RAD analyses. In neighbor-joining tree, individuals of R. tanezumi grouped into geographic-specific branches, and populations from southeast coast were ancestral groups, which confirmed the colonization route from southeast coast to central and western China. However, individuals of R. norvegicus were generally grouped into two clusters instead of geographic-specific branches. One cluster comprised inland populations, and another cluster included both southeast coast and inland populations, which indicated that spread history of R. norvegicus in China was complex; in addition to on-land colonization, shipping transportation also have played great roles. ADMIXTURE and principal component analyses provided further supports for the colonization history. Demographic analyses revealed that climate changes at ~40,000 to 18,000 years ago and ~4000 years ago had led to population declines of both species; the R. norvegicus declined rapidly while the population of R. tanezumi continuously expanded since ~1500 years ago, indicating the importance of interspecies' competition in their population size changes. Our study provided a valuable framework for further investigation on phylogeography of two species in China.

Genomic analysis unveils mechanisms of northward invasion and signatures of plateau adaptation in the Asian house rat

The Asian house rat (AHR), Rattus tanezumi, has recently invaded the northern half of China. The AHR is a highly adaptive rat species that has also successfully conquered the Qinghai-Tibet Plateau (QTP) and replaced the brown rat (BR), R. norvegicus, at the edge of the QTP. Here, we assembled a draft genome of the AHR and explored the mechanisms of its northward invasion and the genetic basis underlying plateau adaptation in this species. Population genomic analyses revealed that the northwardly invasive AHRs consisted of two independent and genetically distinct populations which might result from multiple independent primary invasion events. One invasive population exhibited reduced genetic diversity and distinct population structure compared with its source population, while the other displayed preserved genetic polymorphisms and little genetic differentiation from its source population. Genes involved in G-protein coupled receptors and carbohydrate metabolism may contribute to the local adaptation of northern AHRs. In particular, RTN4 was identified as a key gene for AHRs in the QTP that favours adaptation to high-altitude hypoxia. Coincidently, the physiological performance and transcriptome profiles of hypoxia-exposed rats both showed better hypoxia adaptation in AHRs than in BRs that failed to colonize the heart of the QTP, which may have facilitated the replacement of the BR population by the invading AHRs at the edge of the QTP. This study provides profound insights into the multiple origins of the northwardly invasive AHR and the great tolerance to hypoxia in this species.