What's going on with the golden monkeys in Yunnan, Guizhou and Sichuan?

The golden snub-nosed monkey is a very famous rare primate, and everyone may have heard of it. However, there is actually not only one species of golden snub-nosed monkey, but five species . And not all golden snub-nosed monkeys have golden hair. Among them, the Guizhou golden snub-nosed monkey (Rhinopithecus brelichi), known as the "only child in the world", is a mixture of black and gold. The hair on its head and deltoid tends to be golden, while the hair on its limbs is completely black, and its appearance is very special.

Rhinopithecus brelichi, the gray snub-nosed monkey | Photo: Li Guiyun

Recently, the primate genome project initiated by Chinese scientists published a special research issue in the famous academic journal Science. It not only explained the origin of the black and gold mixed hair of the Guizhou golden monkey, but also found the ancient parents of the Guizhou golden monkey . Moreover, the story behind this is quite tortuous.

Monkey with upturned nose

Golden snub-nosed monkeys belong to the leaf monkey family. The heads of these monkeys tend to look a bit pointy in appearance. They mainly feed on plants and chew leaves every day. The Primate Genome Project reconstructed the diffusion route of leaf monkeys in Asia over the past 8 million years, denying the hypothesis that they entered East Asia from the north, and believed that they went north to East Asia through the southern foothills of the Himalayas and south to Southeast Asia . And about 7.5 million years ago, the strange-nosed monkeys to which the golden snub-nosed monkeys belong began to rise and became a major force among Asian leaf monkeys.

Why is it called the snub-nosed monkey? Because including the golden monkey, the four genera of snub-nosed monkeys together cannot form a perfect nose - yes, the same is true for the golden monkey. Their official name is Rhinopithecus, which means nostrils facing the sky.

The famous Sichuan golden monkey also has its nostrils pointed upwards. Do you think water will enter it when it rains? | Tuchong Creative

The upturned nostrils do not mean that the golden snub-nosed monkeys are arrogant. They just have a bad nose bridge. During the long evolution process, the nose bridge completely degenerated and disappeared , causing the nose to become shorter, so the nostrils look a little upturned, which is conducive to increasing ventilation in plateau areas . In fact, golden snub-nosed monkeys are relatively gentle among monkeys, and they are good-tempered and can form large groups. The primate genome project also explains the emergence of this characteristic of golden snub-nosed monkeys from a molecular mechanism, which may be related to adaptation to cold climates.

Let's talk about the fur color of golden snub-nosed monkeys. In fact, the only species with golden fur is the Sichuan golden snub-nosed monkey (R. roxellana) in Sichuan, Shaanxi, Gansu and Shennongjia, Hubei . It is the first species of golden snub-nosed monkey to be discovered, which is why the golden snub-nosed monkey got its name. Although the Yunnan golden snub-nosed monkey (R. bieti), Nujiang golden snub-nosed monkey (R. strykeri) and Vietnamese golden snub-nosed monkey (R. avunculus) are called golden snub-nosed monkeys, their fur is mainly black or a mix of black and white.

Distribution areas of the five golden snub-nosed monkey species | Image provided by Yu Li's research team and others

The Guizhou golden snub-nosed monkey is the third type, with both golden and black hair . The Guizhou golden snub-nosed monkey is also extremely rare. There are currently more than 2,400 giant pandas, but only about 850 Guizhou golden snub-nosed monkeys. Its distribution area is also extremely limited, only distributed in the Fanjing Mountain area in northern Guizhou, and is known as the "only child in the world."

This makes people wonder, where does the Guizhou snub-nosed monkey's unique hair come from? How did this golden snub-nosed monkey with such a limited distribution originate? What is its relationship with other golden snub-nosed monkeys? How did this species come into being?

Classical breeding and hybrid breeding

In classical evolutionary theory, an ancestral population was separated into multiple independent populations that could not communicate with each other due to migration and environmental changes, and each accumulated different mutations and became different. Later, even when they met again, they could not reproduce fertile offspring, which is reproductive isolation caused by geographical isolation . Based on this, Ernst Walter Mayr believed that the key to defining a species is to see whether a population can be reproductively isolated from other populations, that is, the emergence of reproductive isolation marks the formation of a new species . This argument has been regarded as a criterion by countless people, and has been written into high school textbooks and is well known to people.

It is for this reason that the process of evolution is often described as an evolutionary tree - a tree-like structure in which a common ancestral population continues to branch out over time. This tree-like structure in which each branch does not cross each other shows us that the ancestors of each existing species at any stage of history come from the same population. Today, the species evolutionary tree is one of the most commonly used things by biologists when discussing species evolution.

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Later, people gradually realized that cross-species hybridization is also one of the mechanisms for the formation of new species . Especially in the process of plant evolution, hybrid speciation is considered to be an important mode of speciation, which can usually be divided into three forms: polyploid speciation, homoploid hybrid speciation and introgressive hybrid speciation.

Polyploidy is the process of retaining chromosomes from both parental species, thereby doubling the genetic material and quickly creating reproductive isolation from the parents to form a new species. For example, the tetraploid Primula kewensis comes from the hybridization of two diploid species, P. verticillata and P. floribunda. However, this method can usually only occur in genetic systems with relatively high "compatibility" such as plants, and it almost never occurs in animals. Both homoploid hybridization and introgression do not change the multiples of genetic material, but only present a "mosaic/patch-like" genetic composition due to recombination. During introgression, the offspring will also hybridize with at least one parental species multiple times before a new species is formed.

Hybrid Kew Primula penprimulas

Botanists are familiar with hybridization, but when it comes to animal evolution, zoologists have a different view, and they used to think that hybridization was not very common .

However, as more and more genomic data of species are analyzed, zoologists have found that in many groups, the reproductive isolation between species is not as strong as imagined, and hybridization is very likely to occur. Currently, there are many groups known to have experienced rapid adaptive radiation, that is, a large number of new species exploded in a short period of time, such as butterflies, wolves, baboons, birds and bears.

In this evolutionary process, some species come from the interspecific hybridization of two or more different species, rather than being the descendants of a single ancestral species . At this time, the simple tree structure can no longer accurately describe the evolutionary process of the species.

So, what was the formation process of the golden snub-nosed monkey species? How did the Guizhou golden snub-nosed monkey come into being?

An unexpected but reasonable conclusion

The Guizhou golden monkey is most likely an example of hybridization in nature.

As one of the projects of the Primate Genome Project, the team led by Researcher Yu Li from Yunnan University, in collaboration with several research teams, assembled a high-quality reference genome sequence for the Yunnan snub-nosed monkey and conducted population genetics analysis on the whole genome data of 106 samples of all five species of golden snub-nosed monkeys.

The results showed that the genome of the Guizhou snub-nosed monkey has mixed genetic components from the Sichuan snub-nosed monkey population and the Yunnan snub-nosed monkey/Nujiang snub-nosed monkey ancestral population , which are equivalent to the parents of the Guizhou snub-nosed monkey. However, this is different from ordinary mating. Generally speaking, the ratio of genetic material from the parents of the offspring is close to 1:1, but the genome of the Guizhou snub-nosed monkey has a larger proportion from the Sichuan snub-nosed monkey parent, that is, from the perspective of the genome, the Guizhou snub-nosed monkey is more like the Sichuan snub-nosed monkey . This is probably the reason why it is generally regarded as the sister species of the Sichuan snub-nosed monkey in previous studies.

The researchers conducted a reverse simulation based on the genome, and the results showed that the evolutionary pattern of the Guizhou snub-nosed monkey is likely to be as follows:

About 1.87 million years ago, a hybridization event occurred between the ancestral population of Sichuan golden snub-nosed monkeys and the ancestral population of Yunnan golden snub-nosed monkeys/Nujiang golden snub-nosed monkeys , resulting in the initial Guizhou golden snub-nosed monkey population.

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In later years, the Guizhou snub-nosed monkey, which was of hybrid origin , screened the genetic components of its different parents in order to restore or maintain the stability of the biological functions of the genome.

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In addition, there were two hybridization events between the common ancestors of the Guizhou golden monkey, the Sichuan golden monkey, and the Yunnan golden monkey/Nujiang golden monkey.

So, this is a story that sounds a bit messy, but it explains well why the genetic material from different parental species in the genome of the Guizhou golden monkey is unbalanced .

The evolution of Guizhou snub-nosed monkeys | Photo provided by Yu Li's research team

Next is the question of the hair color of the Guizhou snub-nosed monkey. The researchers found genes related to the formation of the Guizhou snub-nosed monkey's special mosaic hair color. The study found that the color of the golden monkey's hair is determined by the ratio of pheomelanin and eumelanin in melanin . If the proportion of pheomelanin is high, there will be golden hair. If the proportion of pheomelanin is low, there will be black hair. The head and deltoid muscle of the Guizhou snub-nosed monkey have a higher proportion of pheomelanin, which is golden; the limbs have a lower proportion of pheomelanin, which is black. Yu Li and others found five genes related to melanin synthesis in the Guizhou snub-nosed monkey, and they have different origins-SLC45A2, MYO7A and ELOVL4 genes come from Sichuan snub-nosed monkeys, while PAH and APC genes come from the common ancestor of Yunnan snub-nosed monkeys and Nujiang snub-nosed monkeys. It is precisely because of the recombination of melanin synthesis genes from different sources in the genome of the Guizhou snub-nosed monkey that the unique mosaic hair color of the Guizhou snub-nosed monkey is caused.

So how did the Guizhou golden snub-nosed monkey form reproductive isolation from the other two species and eventually become an independent species?

Perhaps it was originally related to hair color . Sexual selection may be one of the explanations - the hair of golden snub-nosed monkeys is different from that of the other two species. A big possibility is that for Sichuan golden snub-nosed monkeys, individuals with golden hair are often more popular with the opposite sex; for Yunnan golden snub-nosed monkeys or Nujiang golden snub-nosed monkeys, individuals with black hair often have a mate selection advantage; and for Guizhou golden snub-nosed monkeys, they prefer individuals with golden heads and black limbs. As a result, pre-mating isolation is formed.

To put it bluntly, they just don't like each other and are not each other's cup of tea. Pictured is the Yunnan golden monkey｜TuChong Creative

In addition to isolation before mating, isolation after mating may also promote the evolutionary independence of the hybrid-origin Guizhou snub-nosed monkey and its two parents . For example, in terms of genes related to reproductive traits, 31 genes inherited from Sichuan snub-nosed monkeys and 36 genes inherited from the other parent are retained in the Guizhou snub-nosed monkey genome. As a result, the Guizhou snub-nosed monkey may cross-establish post-mating isolation with the two parents based on these two reproductive-related functional gene sets, and eventually form an independent species.

The evolution of the Guizhou snub-nosed monkey proves that mammals can form new species through hybridization. The success of this study also shows that the primate genome project initiated by Chinese scientists has unlimited potential. There are more than 500 primate species in the world. Research on them can help us learn more about the origin, evolution, physiological characteristics, genetic variation maps, and disease-causing gene variations of humans .

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Author: Ran Hao, Huang Zijian

Editor: You Shiyou, Heijiomeimei

Thanks to teachers Yu Li and Wu Hong for their revision suggestions.

This article comes from GuokrNature (ID: GuokrNature)

If you need to reprint, please contact [email protected]

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