Monkey stories: The Three Kingdoms of Sichuan golden monkeys

Produced by: Science Popularization China

Author: Bai Lu, Zhao Xumao (College of Ecology, Lanzhou University)

Producer: China Science Expo

As a species unique to China, Sichuan golden snub-nosed monkeys are classified as "national treasures", but in the past 400 years, due to the increase of human activities and other reasons, their numbers have dropped dramatically. They are currently distributed in the high mountains and dense forests of Sichuan, Hubei, Shaanxi, and Gansu. The Sichuan golden snub-nosed monkeys in these four provinces can be divided into three "mountains", namely the Shaanxi-Gansu group, the Qinling group, and the Shennongjia group.

At the end of the Eastern Han Dynasty, China was divided into three kingdoms. The formation of the Three Kingdoms pattern was due to the competition between the three powers of Wei, Shu and Wu. The formation of the Three Kingdoms (three genetic lineages) of Sichuan golden snub-nosed monkeys is due to geographical isolation. First, the Shennongjia population was isolated from Sichuan golden snub-nosed monkeys in other regions about 245,000 years ago, leading to its independent evolution. Second, 20,000 years ago, the Shaanxi-Gansu population and the Qinling population were distributed in the Minshan Mountain Range and the Qinling Mountain Range. The two developed independently due to the long-term lack of genetic exchange.

Since then, although the Sichuan golden snub-nosed monkey is one species, it has been divided into three independently evolving genetic units, forming a tripartite situation. Today, these three lineages are hundreds of kilometers apart and seem to have been isolated for at least 10,000 years.

Sichuan golden monkey

(Photo credit: Photographed by Zhao Xumao)

The three-legged tripod is not just nonsense!

The seemingly simple "three-legged tripod" situation requires rigorous scientific basis. So how did researchers prove from the perspective of molecular biology that these three geographical units evolved independently?

Geographical distribution of Sichuan golden snub-nosed monkey

(Image source: provided by the author)

This requires the use of genome data from three Sichuan golden snub-nosed monkeys from different geographical units, by comparing their genetic distances and genetic differences. According to the size of the differences, it is determined how many genetic units they can be divided into.

Sichuan golden monkey

(Photo credit: Photographed by Zhao Xumao)

All things in the world, will eventually separate after a long period of union?

The next question is, what caused the differentiation of the three genetic units of Sichuan golden snub-nosed monkeys, and what was the differentiation process like?

Although we cannot travel through time, we can rely on species distribution models to trace their geographical distribution in history and thus get a glimpse into the differentiation process between them. Species distribution models simulate the distribution of species based on the relationship between species distribution and the environment.

The distribution of species is closely related to the surrounding environmental conditions, and the range of behavioral, biological, social, and ecological factors that cause a species to expand or contract its range and population size under changing climate conditions are the main focus of conservation biology.

Once we know this connection, we can use today's environmental data and current species distribution data to simulate the suitable distribution area of ​​the species. Similarly, based on the relationship between species distribution and environment, we can replace the current environmental data with environmental data from historical periods and trace the species distribution in historical periods.

I understand the principle, but where should I start the story?

Based on the molecular clock of the Sichuan golden snub-nosed monkey genome, we can calculate the approximate differentiation time of the three geographical units of the Sichuan golden snub-nosed monkey. In layman's terms, the molecular clock means that the evolution of species is constant at the molecular level. Based on this characteristic, we can calculate the differentiation time of different species from the ancestral species. Similarly, we can infer the differentiation time between different genetic units of the same species by analogy.

From this, we know specifically that the approximate differentiation time period of the three genetic units of the Sichuan golden snub-nosed monkey was during the last glacial period and the interglacial period.

With this time point, we can use the species distribution model to simulate the changes in the distribution range of Sichuan snub-nosed monkeys during the last interglacial period, the last glacial period, and now. Our simulation results show that:

During the last interglacial period, the distribution of Sichuan golden snub-nosed monkeys was connected, in which case they could exchange genes with each other and were not likely to differentiate; however, during the last glacial period, the distribution of Sichuan golden snub-nosed monkeys shrank, and the three main distribution areas lost contact, resulting in a break in gene exchange between them, and they all began to undergo adaptive evolution. Therefore, different genetic geographical units were formed.

At the same time, what changes occurred in the population size of Sichuan golden snub-nosed monkeys during the last interglacial period and the glacial period?

Generally speaking, the effective population size of the Sichuan golden snub-nosed monkey changes with the distribution area. When the distribution area expands, the population size also expands. Of course, there are also special cases. We can still use genome data to estimate the changes in the effective population size of the Sichuan golden snub-nosed monkey in the historical period.

This not only allows for mutual verification with changes in distribution areas, but also allows for a clearer understanding of the differentiation between them. Our simulation results show that during the last glacial period, the effective population size of Sichuan snub-nosed monkeys shrank, which was basically consistent with the changes in distribution areas. However, population changes between different geographical units are different.

The combination of distribution changes and population changes between different geographical units is the main reason for the current differentiation of Sichuan golden snub-nosed monkeys.

Aside from genetic differences, what else is different about them?

According to the observations of Li Baoguo's team at Northwest University, Sichuan golden snub-nosed monkeys in different geographical units have certain differences in social structure, sex ratio, behavioral habits, etc. For example, Sichuan golden snub-nosed monkeys are "polygamous", and the adult sex ratio in a breeding family, or in layman's terms, the number of spouses of the main male monkey, is different. The Qinling unit is 4.27:1, the Shaanxi-Gansu unit is 5.3:1, and the Shennongjia unit is 6.96:1.

Since there are differences between different genetic geographical units, how should we protect them? The ultimate goal of conservation biology research is to provide some suggestions and measures for the protection of species. With the support of species distribution models, we can predict what changes will occur in the distribution of Sichuan golden snub-nosed monkeys under future climate change conditions.

Traditional research uses species distribution models to infer the future suitable distribution area based on the distribution of Sichuan golden snub-nosed monkeys. However, this has certain problems. As we mentioned earlier, different geographical units have already undergone some differentiation, so their demands for the environment have also changed accordingly.

Based on this, we constructed species distribution models for different genetic geographical units under the same species, that is, we simulated them separately according to the climate requirements of different genetic units, so that we can more accurately treat the Sichuan golden snub-nosed monkey as a whole.

Therefore, our first step is to use spatial models to assess the changes in the environmental requirements of the three geographical units of the Sichuan snub-nosed monkey. We found that their requirements for 19 bioclimatic changes varied between different genetic geographical units, which is a prerequisite for in-species modeling.

Therefore, we simulated the response mechanisms of three different genetic geographical units under future climate change conditions. The results showed that under future climate change conditions, the Shennongjia unit is more vulnerable. In this way, we can delineate priority protection units and carry out targeted and predictive protection. This project was completed by Zhao Xumao's research group from the School of Ecology, Lanzhou University and Li Ming's research group from the Institute of Zoology, Chinese Academy of Sciences. The research paper was published in the international classic journal Diversity and distributions.

In the world of Sichuan golden snub-nosed monkeys, although there are three major groups, there is no fighting. It is a happy thing for them to "live in peace" among the mountains and dense forests!

Editor: Sun Chenyu

References:

Zhao, X., Li, https://doi.org/10.1111/ddi.1363

(Note: Latin parts in the text should be italicized)