An epidemic has made us particularly impressed by this year of the rat, so on the "tail" of the year of the rat, we will talk about the "rat" as one of the important pathogenic animals.
People often say that "rats" generally refer to rodents, which are the taxons with the largest number of extant species among mammals. They include rodents and rabbits. Rodents are the food source of many carnivores, so they are a key link between primary producers (plants) and secondary consumers (carnivores) and are an important part of natural ecosystems.
Figure 1 Prediction of the future composition of mammalian lineage diversity (Davis et al., 2018)
So, how did rodents, as the largest family of mammals, develop and prosper? Studies have shown that a large number of large mammals have experienced a large-scale distribution reduction or even extinction due to climate change and the rapid expansion of human society since the late Quaternary period. However, small and medium-sized mammals such as rodents, bats, and carnivores are at low risk, and projections show that rodent diversity will continue to increase in the future (Figures 1, 2). This suggests that they may be one of the largest beneficiary taxa during the decay of large mammals.
Figure 2 Several small and medium-sized mammals in the current natural ecosystem
A. Rhinolophus macrotis, B. Chimarrogale leander, C. Plateau Rabbit (Lepus oiostolus), D. Himalata himalayana
Lurking in the forest to "secretly observe" the long-tailed family of humans
The first impression that rodents give to the public is not very good. As one of the "four pests", they not only steal our food and fruits, but also spread viruses that can infect humans. This type of animal, the plague pathogen that has historically ravaged Europe, was transmitted by this type of animal. In recent years, circulating epidemic haemorrhagic fevers in the country have also been induced by hantavirus carried by rodents and other small mammals.
Every time an epidemic occurs, the CDC will rush to the scene as soon as possible to sample the environment at the source of the epidemic, including wild animals, to determine the source of infection. To the surprise of the researchers, over the course of a long investigation, they found that in the wild environment over a vast geographical range from southwest China to northern China, there is a class of rodents that are not only large in number but also diverse in form, and have been found in these animals many times.
This class of animals has some significant characteristics that distinguish them from the common species of the genus Of Hordes (brown rats, yellow-breasted rats, etc.), that is, most of them have white abdomen, the length of their tail exceeds the length of their bodies (Figure 3), they have strong climbing ability, although they mainly live in forest environments, rarely appear in human homes, but they are often found in bushes, plantations, etc. near human habitats.
Fig. 3 Morphological characteristics of species of the genus White-bellied rat
A. Niviventer lotipes, B. South China needle rat (N. huang)
So, the question is, what kind of animals are these? Where did their ancestors come from, and how did they develop and flourish? Which of them are the most potential threats to humans? In order to solve these problems, researchers have carried out a large number of basic research work, the main work is divided into the following aspects:
Family members and family relationships
By studying a large number of specimens, the researchers found that this long-tailed little guy was the leader of the rodent family in southeast Asia to China, the genus Niviventer J. T.Marshall,Jr., 1976）。 In order to figure out the species composition of this taxon, how many species are distributed in China, and what is the relationship between these species, researchers at the Institute of Zoology of the Chinese Academy of Sciences first conducted taxonomic and systematic studies on this taxon. At least 17 species of the genus were found to be distributed in China, two of which were new species from southeastern Tibet, namely the sword-striped gladiusmaculus and the Feng's white-bellied rat (Niviventer fengi).
To further elucidate the relationship between these 17 species, the researchers further reconstructed the interspecific relationships within the genus White-bellied Rat with the help of genomics data, which turned out to be divided into four species clusters:
Grey-bellied rat species ("grey-bellied" family): There are two very specific species in the genus White-bellied Rat, the Grey-bellied Rat and the Van Rat, whose belly is gray-white.
Ahn's White-bellied Rat Species ("Giant" Family): Includes the two largest species in the genus Ahn's White-bellied Rat, the Ann's White-bellied Rat and the Chuanxi White-bellied Rat.
She-rat breed group ("winning" family): Includes the largest and most widely distributed species in the genus White-bellied Rat, two of which are endemic to Taiwan. (Figure 4)
Needle hair rat species ("needle hair" family): The first species named in this species group has needle hair, so it is called needle hair rat, and some of them do not actually have needle hair characteristics. If you do not consider geographical distribution, genetic distance and other factors, it is easy to make mistakes based on the presence or absence of needle hairs. (Figure 5)
Figure 4 Morphological characteristics, distribution and phylogenetic relationship between white-bellied rat species
Figure 5 Morphological characteristics and distribution of needle-haired rat species in the genus White-bellied rat
Therefore, the Chinese distribution of the large family of the white-bellied rat genus consists of at least 17 small families, which constitute 4 larger tribes (Figure 6). Calculations based on molecular clocks show that this genus began to differentiate around 5.52-4.33 million years, and the population of Beishe rat, South China needle-haired rat, and gray-bellied rat has shown significant population increase in history (Figure 7).
Figure 6 Phylogenetic relationships between species of the genus White-bellied Rat
Figure 7 Differentiation time and evolutionary history of populations of the genus White-bellied rats
Where do they come from? Where are they going?
After figuring out their phylogenetic relationships, then next, it is necessary to figure out where they originated from and how to diffuse to form the current distribution pattern. Today's biological research often requires the help of many mathematical algorithms, building models, and making calculations. The process of reconstructing ancestral distribution sites is based on the phylogenetic relationships between species and where they are now distributed to infer the distribution of their common ancestors. Based on this analysis, the ancestral distribution area of the genus White-bellied Rat was deduced.
The southeastern region of the Qinghai-Tibet Plateau is estimated to be the ancestral distribution site of the white-bellied rat. There are four patterns in the evolutionary history of biogeography within the genus White-bellied Rat (Figure 8): the Grey-bellied Rat Species Remains on the SoutheastErn Edge of the Qinghai-Tibet Plateau; the Ann's White-bellied Rat Species Has Spread from the Qinghai-Tibet Plateau to the Surrounding Mountains of the Sichuan Basin, the Qinling Mountains and the Wuling Mountains; The Needle-haired Rat Species Has Spread from the SoutheastErn Edge of the Qinghai-Tibet Plateau to Southeast Asia and Southeast China many times; and the most successful Rat Species Have Spread Not Only to Southeast Asia, Southeast China, but also north to Northern China. The above speculation is supported by a combination of mitochondrial genome and nuclear genomic data.
Thus, the home of the genus is the southeastern region of the Qinghai-Tibet Plateau (about 2100 meters above sea level, Figure 9), and they have left their homeland and expanded everywhere, and the families that have traveled the farthest have "climbed" to southeastern and northern China and even Southeast Asia.
Figure 8 Biogeographic evolutionary history of the genus White-bellied rat
Figure 9 Evolutionary history of the distribution altitude of the genus White-bellied Rat
Who are they most closely related to? And who is the parent, and there is a crossover occurring?
In today's research on biosystemology, some scholars have proposed that the original binary topology is not enough to indicate the phylogenetic relationship between species, which requires the analysis of the relationship between phylogenetic networks. Such phylogenetic network analysis can not only elucidate the kinship between species, but also discover the timing, intensity, and direction of gene flow between species. The results revealed that there may be multiple hybridization events within the genus Chiromyscus, which is interspecific within the genus White-bellied Rat and the nearest margin.
Figure 10 Phylogenetic network diagram of the genus White-bellied rats
Which family excels in exploring new territories?
The ability of different species to occupy ecological space is not the same. So which taxa have been able to win over their long evolutionary history depends largely on their ability to quickly occupy different niches. Based on the analysis of niche differentiation, the researchers found that needle hair rat and social rat species were excellent, showing rapid differentiation in terms of various environmental factors analyzed. This also provides evidence for later explanations of why they are likely to expand rapidly in the future.
Figure 11 History of niche differentiation of the main representative species of the genus White-bellied rat
Who is likely to continue to expand their territory in the future?
Based on the existing distribution location information of the species, researchers can speculate about their potential habitat changes over different geological historical periods and even in the future under different environmental conditions. This is important for important source animals, because knowing their future expansion trend, humans can prepare for it as early as possible. Projections of the genus White-bellied rats show a tendency for most species with high- and medium-altitude distribution to retreat in their potential habitats in the future, while species with the widest distribution and species from Southeast Asia show a tendency to expand (Figure 12).
Interestingly, the study also found that the overall distribution area of the gray-bellied rat and the white-bellied rat in western Sichuan, which is endemic to the Qinghai-Tibet Plateau, may be reduced in the future, but they will further expand to the plateau surface, and the average elevation of the distribution will be further increased by 200-800 meters.
Figure 12 Potential habitat changes of the main representative species of the genus White-bellied Rat
Which species is actively modifying its genetic base to adapt quickly to the environment?
For different biological groups, there are often different strategies for adapting to the environment, the most common of which is the adjustment and change of morphology, behavior and physiological adaptation. Changes in morphology and behavior can be detected through close observation with the naked eye, but physiological changes are more difficult to observe with the naked eye. The more common method now is to scan the genome of the species studied. The researchers also wanted to figure out which species in the genus White-bellied Rat is undergoing rapid changes in the genome, improving autobiotic adaptation by accumulating favorable mutations. Before doing this analysis, they came up with the following conjectures:
Hypothesis 1: Species with the largest distribution range are subject to the most positively selected genes, i.e. species that can occupy a wider ecological space will be actively modified on the genome to adapt to a wider environment.
Hypothesis 2: The largest or smallest species are subject to the most positively selected genes, i.e., species with greater morphological variation may have the most positively selected genes in their genomes compared to closely related species of the same genus.
Hypothesis 3: Species with the highest distribution altitude are subject to the most positively selected genes, that is, the living environment is the most severe, and the species distributed in the Qinghai-Tibet Plateau are most positively selected.
The results of the analysis are consistent with the third hypothesis, namely that gray-bellied rats distributed on the Tibetan Plateau are most positively selected by genes (Figure 13). Only one gene of the sister species of grey-bellied rats, van rats, was positively selected, while Von's white-bellied rats, which have a certain distribution overlap with gray-bellied rats, had five genes that were positively selected. The main functions of these genes are related to smell, taste, and cancer suppression.
Figure 13 Appearance characteristics of gray-bellied rats (Source: Liu Shaoying et al., editors of the "Atlas of Chinese Mammals")
In the past, Chinese scientists have found that the ancestors of cold-loving animals such as woolly rhinoceros, leopards, foxes, and proto-sheep are distributed on the Qinghai-Tibet Plateau based on a large number of fossils, and in the living cold-loving animal group, the pika is also a typical representative of "walking out of the Qinghai-Tibet Plateau". The results of the study on the genus White-bellied Rats show that "out of the Qinghai-Tibet Plateau" occurred not only in the evolutionary history of cold-loving animals, but also in the evolutionary history of white-bellied rats that can adapt to temperate and tropical environments. Rapid evolution of genes associated with smell, taste, and inhibition of cancer may play an important role in rodent altitude adaptation. In addition, population growth and potential future habitat expansion of generalized species in Southeast Asia and habitats may pose potential risks to ecosystem balance and human health.
There is still a long way to go for humans to understand the mouse family
In the midst of the hustle and bustle, the Year of the Rat is coming to an end. But human understanding of rodents still has a long way to go. Many taxa of rodents distributed in China are in urgent need of systematic research. The genus White-bellied Rat is only one of the representative genera that researchers are studying, and there is still a lot of work to be done.
The bell of the Year of the Ox is about to ring, and researchers will continue to explore the mysterious rodents.
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Source: Institute of Zoology, Chinese Academy of Sciences