1. Introduction
Rabbits have been domesticated for thousands of years [
1]. The process can be divided into two stages: the first stage was from wild to domestic animals, and the second one is from domestic animals to landraces and even improved breeds. We identified genomic regions influenced by domestication in our previous study in which the genomes of wild rabbits (from the Iberian Peninsula and southern France) and domestic populations (from Europe and China) representing different geographic and genetic origins were compared. In this study, we wanted to explore the genetic diversity and genomic signature during the second stage, when the rabbits showed more distinct morphological and economic traits, and selective signatures were expected to be present in the genomes of different breeds due to adaptation to a diverse range of environments and specialized production systems during breeding and improvement.
Rabbit breeding holds significant global importance due to its contribution to food security, sustainable agriculture, and the development of new pharmaceuticals, with over 300 recognized rabbit breeds (
https://www.fao.org/dad-is/dataexport/en/, accessed on 5 November 2023) worldwide reflecting a rich genetic diversity. This diversity provides an invaluable resource for genetic research, offering insights into genetic mechanisms underlying phenotypic variation, disease resistance, and adaptation to different environments. The extensive use of rabbits in genetic studies has also facilitated advancements in understanding complex genetic diseases in humans, making them a vital model organism in biomedical research [
1,
2]. There are at least 40 indigenous, cultivated, and recently introduced rabbit breeds in China, which are primarily distributed in Shandong, Sichuan, Henan, and Hebei [
2]. Chinese indigenous rabbit breeds are characterized by their small body size and slow growth rate. However, they have excellent meat quality, strong resistance, and a high reproductive capacity. Some studies have been conducted on the genomic selection characteristics and major effect genes related to common traits in Chinese domestic rabbits. Coat color is the most prominent feature of domestic rabbits, and white rabbits are currently the most common and widely bred breed of domestic rabbit.
MC1R,
ASIP,
TYR, and
KIT are the important genes that influence coat color in rabbits [
3,
4,
5]. The black spots on the coats of Checkered Giant rabbits are caused by mutations in
MC1R [
3].
ASIP controls the production of agoutis, which results in the gray-brown or gray coloration of dorsal fur [
4]. Tyrosinase (TYR) is the rate-limiting enzyme required for the synthesis of melanin in melanosomes. Mutations in this gene can alter or impair the function of the encoded enzyme, leading to the occurrence of certain forms of albinism in domestic rabbits [
6]. In addition, different breeds can also be distinguished by their hair structure and length. Natural mutations in the
LIPH gene have been shown to be the cause of hair growth defects in humans and the Rex short-haired phenotype in rabbits [
7]. Mutations in
FGF5, a regulator of the hair cycle, can cause an inherited long-haired phenotype in various species, including domestic rabbits, resulting in an overall increase in hair length [
8].
To date, high-density marker data, restriction-site associated DNA sequencing (RAD-seq), and whole-genome sequencing have been used to study selection signals in different rabbit breeds. For example, Carneiro et al. utilized whole-genome pooled sequencing to reveal the multi-gene basis of phenotypic changes during the domestication process [
9]. Mohamad et al. identified genes associated with coat color, coat structure, and body size in meat and high-quality rabbit breeds based on high-density marker data [
10]. Liu et al. explored the genomic resources of native Chinese indigenous rabbit breeds using RAD-seq technology and identified genes associated with melanin synthesis [
11]. The development of next-generation sequencing (NGS) technology has utilized the high-throughput detection of Single Nucleotide Polymorphism (SNP) molecular markers, contributing to the study of animal genetics at the genomic level [
12]. With the decrease in the cost of high-throughput sequencing, an increasing number of studies have started to utilize this technology for research on genomic diversity and selection features. In pigs, cattle, sheep, chickens, and other livestock and poultry, a large number of whole-genome sequencing studies have been used for the analysis of selection signals [
13,
14,
15,
16].
Our study bridges critical knowledge gaps by distinguishing between previously identified genes and uncovering new genetic markers associated with key adaptive traits in rabbits, including coat color, structure, long hair, body size, reproductive capacity, and disease resistance. By integrating findings from past research with our novel discoveries, we significantly enhance the understanding of rabbit genetics. This dual approach not only validates the contributions of earlier studies but also expands the genetic framework underlying rabbit adaptability and phenotypic diversity, offering a more comprehensive insight into their evolutionary development and potential for selective breeding enhancements. In the current study, the key research topic is as follows: to explore the genomic signatures underlying important morphological, production, and adaptive characteristics in Chinese rabbits during the breeding and improvement, we compared genomes of diverse domestic rabbit breeds to detect genetic diversity and positive selection traits that may explain their phenotypic variations.
4. Discussion
In this study, we utilized whole-genome resequencing to obtain a vast number of genomic variations, elucidating the genetic backgrounds of different rabbits. The results showed that JYS and LWB exhibited the highest genetic diversity. This may be the result of hybridization with other breeds [
11,
72]. The genetic diversity of JW and WHHL rabbits was significantly lower than that of other rabbits, which is consistent with a recent report [
17]. JW and WHHL were genetically distant from other rabbit breeds, which is consistent with their breeding history and may be related to the process of artificial selection [
17].
Through selective sweep analyses, we identified several selective signatures in the rabbit genome. Some of the results we obtained highlighted genes previously identified to influence coat color (
TYR) [
11], pigmentation-related traits (
ASIP) [
4,
10], and coat structure (
LIPH) [
7,
10,
73]. This demonstrates that the methods we employed are capable of capturing recent selective features. We found strongly selected regions on the genome in both the coat color and coat structure variations. These regions contain key genes that affect the color (
TYR) and structure (
LIPH) of rabbits’ coats.
TYR is a crucial gene in the process of melanin formation, and its variations lead to allelic series of white-coat-color loci [
5], highlighting the breed-specific characteristics in this region. Variants located upstream of the
GRM5 gene have independent effects on pigmentation and may potentially regulate the expression of
TYR [
74]. The expression of
NOX4 can inhibit the expression of the tyrosinase gene, thereby reducing the production of melanin [
75].
RAB38 regulates the transport of melanogenic enzymes, particularly tyrosinase, from the trans-Golgi network (TGN) to melanosomes [
76].
NOX4,
GRM5, and
RAB38 have not been reported to have an impact on coat color in rabbits to date, but they all have the potential to influence the expression of tyrosinase genes [
74,
75,
76], which control fur color in domestic rabbits.
ASIP is a widely studied pigmentation gene that plays an important role in melanin synthesis and is associated with the darkness or lightness of an animal’s coat color [
77,
78,
79].
RALY and
ASIP contribute to the number of facial pigmented spots acquired during the aging process through pathways independent of basal melanin production [
80]. ASIP and RALY can also influence the coat color of horses and the deposition of facial pigmentation in humans [
79,
80]. They are important candidate genes that determine variations in coat color in domestic rabbits.
In the upstream region of
LIPH, we observed intense selection acting on the unnamed genes and the region where long non-coding RNAs are located. The variation in coat color and structure of rabbits may be due to a mutation in a linked region. Our research has revealed the close association of
LIPH,
FPB41, and
WASF2 with hair follicle development.
LIPH specifically influences the cortical structure impacted by the REX locus [
7,
73].
EPB41 is closely linked to defects in hair follicle structure and function, as well as skin lesions and tumor development [
81]. Additionally,
WASF2 acts as a critical regulator of epidermal shape and growth [
82].
Furthermore, we also identified genes related to hair growth and hair follicle development (
MSX2,
CERS6,
HDAC9,
RASA1, and
CLDN18) in the selection signals of Angora rabbits, which may provide new insights into the development of hair. HDAC gene family members play a significant role in modulating the structure of chromatin and act as crucial regulatory factors in gene transcription. They are closely associated with human baldness [
83].
MSX2 is involved in the processes of hair shaft differentiation and hair follicle neogenesis [
84,
85].
CLDN18 plays a critical role in the differentiation process of the epidermis and hair follicles [
86].
RASA1 promotes the proliferation of goat hair follicle stem cells and inhibits cell apoptosis [
8].
The complexity of the selection signatures observed in comparisons between small-, medium-, and large-sized rabbit breeds indicates that a multitude of genes influencing growth and development are involved in determining rabbit body size and productive performance. This finding is similar to reports in other mammals.
INSIG2,
GLI3, and
NRXN3 are known to affect human body mass index and height [
35,
38,
47].
LGR4 and
PRKCQ have been found to be closely associated with body weight in studies of pigs and humans [
44,
49,
50]. These are potential candidate genes that may contribute to the differentiation of body size in domestic rabbits.
In the exploration of genetic selection signals among rabbits with diverse reproductive capabilities, we have pinpointed several genes that play pivotal roles in reproductive traits, the development of sexual glands, and the onset of sexual maturity. These genes include SST, which is known to modulate the expression of genes crucial for fertility, as well as the maturation of oocytes and the development of embryos, highlighting its significant role in reproductive success [
56]. EDNRA and ACOXL have been identified as potential markers for fertility in cattle [
2], suggesting their broader importance in reproductive biology across species. These genes are implicated in essential reproductive processes, with EDNRA involved in the endothelin signaling pathway that influences vascular functions and follicular development, and ACOXL potentially linked to metabolic pathways critical for reproductive energy demands. GLI3 and TGFB2 are integral to the development of sexual glands [
53,
54,
58], with GLI3 affecting the Sonic Hedgehog signaling pathway crucial for organogenesis, including gonadal development, and TGFB2 part of the transformative growth factor-beta pathway, known for its role in cellular differentiation and reproductive organ development. Lastly, PLCB1 has been associated with central precocious puberty and is crucial for the development of external genitalia, underlining its significance in reproductive maturity [
51,
52]. These discoveries not only shed light on the genetic underpinnings of reproductive traits in rabbits but also resonate with known mechanisms in other species, suggesting a conserved genetic framework underlying reproductive biology. This enhanced understanding opens avenues for further research into reproductive health and fertility across species, offering potential strategies for managing fertility and understanding reproductive disorders.
The identification of genes linked to environmental adaptation and immune responses in Chinese indigenous rabbits, as compared to foreign domestic rabbits, holds significant practical implications for adaptability and health across diverse environmental conditions. Zhang et al., through their study on the genetic variation in resistance to bacterial infection in growing meat rabbits, found that the incidence and mortality rates of bacterial infections in Chinese indigenous rabbits were lower than those in California rabbits, Belgian rabbits, and Chinchilla rabbits [
1]. Chen et al., through their research on the nonspecific disease resistance characteristics of Chinese local domestic rabbits, discovered that they possess good nonspecific disease resistance [
1,
2]. Specifically, genes such as
PLCB1,
LAP3,
SEC31A,
CD86,
IL6R,
ISL1,
GSK3B, and
PLD1 play pivotal roles in biological processes crucial for survival and adaptation. For instance, PLCB1 is integral to intracellular signal transduction, facilitating responses to a variety of extracellular signals. Its association with heat tolerance in other species suggests its potential in aiding Chinese indigenous rabbits to adapt to warmer climates [
62,
63]. Similarly, ISL1, which is crucial for heart development, might contribute to the high-altitude adaptability seen in some rabbit populations, mirroring findings in cashmere goats [
65,
66]. GSK3B’s involvement in hypoxic condition adaptation suggests that rabbits with this gene could better withstand low-oxygen environments [
64].
PLD1 plays a pivotal role in the autophagy process [
67,
87] and acts synergistically as a virulence factor, thereby significantly contributing to the invasion of host cells [
68].
LAP3,
CD86, and
SEC31A play important roles in the immune system [
69,
70,
71]. These genetic traits not only highlight the potential for selective breeding programs to enhance environmental resilience and disease resistance in rabbits but also underscore the importance of conserving genetic diversity within indigenous populations. Furthermore, understanding these genetic mechanisms offers a valuable model for biomedical research, potentially advancing our knowledge of human adaptability and immune response, and paving the way for novel therapeutic strategies.
Overall, the study of these genes not only sheds light on the genetic foundations of adaptability and health in Chinese indigenous rabbits but also opens avenues for practical applications in breeding, conservation, and biomedical research, emphasizing the critical role of genetic diversity in the resilience of animal populations to environmental and health challenges.
This study, while offering valuable insights into the genetic diversity and specific genes associated with adaptability, health, and phenotypic traits in rabbits, has its limitations, including the scope of genetic diversity covered, direct correlation between genetic markers and phenotypes, and the consideration of environmental interactions. Future research could address these limitations by expanding the genetic sampling across more rabbit breeds and wild populations, conducting functional genomic studies to elucidate the mechanisms behind gene-trait associations, exploring gene-environment interactions, and undertaking longitudinal studies to observe genetic changes over time. Such research endeavors would not only deepen our understanding of rabbit genetics but also enhance breeding programs and conservation efforts, and provide broader insights into evolutionary biology and genetics.
5. Conclusions
In our comprehensive exploration of the genetic diversity and population structure among Chinese indigenous rabbit breeds and foreign breeds through whole-genome resequencing, we discovered that the LWB, JYS, and REX breeds exhibit notably higher genetic diversity compared to others. Specifically, LWB and JYS breeds are distinguished by their multiple genetic ancestries, indicating a rich and complex genetic background that contributes to their unique characteristics. Further analysis revealed that a significant portion of the genetic ancestry of LWB and JYS breeds comes from the New Zealand White (NZW) rabbits, with a smaller contribution from the Fujian White (FJW) and other domestic rabbit breeds. This mixture of ancestries suggests that LWB and JYS rabbits have inherited a broad range of genetic traits, which may account for their distinct phenotypes and adaptabilities. The presence of NZW ancestry in particular could be linked to traits such as size, growth rate, and fur quality, while the influence from FJW and other breeds may add to their genetic diversity, potentially enhancing their resilience and adaptability to various environments. Our investigation into the selection signatures across diverse rabbit populations has unveiled multiple regions within the genome that show significant selection signals, particularly in traits related to coat color, coat structure, and hair growth. These findings not only enrich the existing pool of candidate genes associated with these traits but also highlight specific genes under strong selection in Chinese indigenous rabbits, underscoring their potential role in defining the breed’s distinctive features.
Building on these insights, we recommend future breeding endeavors to focus on leveraging the identified genetic diversity and selection signatures to enhance breed characteristics such as coat quality, color, and body size. Breeders should consider the unique genetic attributes and ancestral components of the LWB, JYS, and REX breeds when designing breeding programs, aiming to preserve or enhance these valuable traits. Additionally, the strong selection signals identified in Chinese indigenous rabbits offer a promising avenue for developing breeds with enhanced adaptability and distinct phenotypic traits.
Conclusively, our findings lay a foundational stone for the future of rabbit breeding, conservation efforts, and genetic research. By illuminating the genetic underpinnings that contribute to the diversity and distinctiveness of Chinese indigenous and foreign rabbit breeds, this study not only aids in the conservation of these breeds but also paves the way for informed breeding strategies. The insights gained from our research hold the potential to significantly influence the direction of future breeding practices, ensuring the preservation of genetic diversity and the enhancement of desirable traits in rabbit populations worldwide.