Rumpless chickens

Rumpless chickens are a group of poultry breeds characterized by caudal dysplasia, a condition that results in the absence of a tail. This trait involves the lack of the pygostyle, which is the small tailbone that supports tail feathers in most birds and is also referred to as the caudal appendage or the "parson's nose". This unique characteristic has been observed to be autosomal dominant trait in some breeds and recessive in others, but both resulting from a genetic mutation. This mutation is currently unknown, but multiple candidates have been proposed. Rumplessness in chickens has sparked interest in genetics and developmental biology, particularly in studies that focus on the mechanisms of tail development.

History

"Gallus ex Persia", illustration from the Ornithologiae Tomus Alter of Ulisse Aldrovandi, 1600

Rumplessness in chickens has been documented for centuries. An early description is that of the Bolognese ornithologist Ulisse Aldrovandi in his Ornithologiae Tomus Alter of 1600. While the rumpless trait was observed in different breeds across the world for centuries, it became more widely recognized in the 19th and 20th centuries, with poultry breeders and geneticists starting to explore the inheritance of unique traits such as rumplessness. The scientific study of rumplessness intensified after the development of modern genetics, leading to a better understanding of genetic mutations. These studies continue today.

Early breeders sought to preserve and refine these breeds for their distinctive appearance. For example, both the Araucana and Manx Rumpy became symbols of regional pride and were protected by breeders eager to maintain their genetic legacy. While the exact origins of the rumpless trait remain somewhat mysterious, the scientific understanding of the genetic mutations behind it has evolved, paving the way for more breeding and conservation efforts in modern times.

Illustration from the Geflügel-Album of Jean Bungartz, 1885

Rumpless breeds

Rumpless breeds of chicken include: the Araucana or Rumpless Araucana of South America, both large fowl and bantam; the Belgian Barbu de Boitsfort, Barbu de Grubbe and Barbu d'Everberg bantams; the Japanese Uzurao, a rumpless equivalent of the Tosa-Kojidori bantam, and Ingie (large fowl) from Kagoshima Prefecture; the Rumpless Game of the United Kingdom (both large fowl and bantam), sometimes called the Manx Rumpy or Persian Rumpless; the German Ruhlaer Zwerg-Kaulhühner or Rumpless Booted Bantam; and the Piao of China, also called the Dongxiao.

Physical traits

Rumpless chickens, for the most part, possess the same physical body structure as other breeds; the primary difference lies in the lack of pygostyle development. These birds are left without a tail, providing the basis for the "rumpless" name. This characteristic differs from most chickens, as these do not possess tail feathers at all, providing the distinctive rumpless look. The remainder of the physical traits of rumpless chickens are determined by their breed. This includes plumage, which varies in color and length based on the breed. For example, the Araucana has a blue egg-laying characteristic and tufted ear feathers.

Genetics

The mutation inhibiting pygostyle development occurs occurs differently among different breeds. This mutation has been noted to affect the notch signaling pathway, which plays a vital role in tail development during the embryonic stage of birds. This trait is expressed differently across breeds, but the absence of a tail remains consistent as the key physical marker. The mutation has been studied most in the Araucana and Piao breeds.

In Araucana chickens, rumplessness is caused by a dominant autosomal genetic mutation known as rumplessness (Rp). This mutation is located on an autosomal gene and causes the birds to be developed without a tail. The Rp gene for rumplessness is dominant, and thus only one copy of the Rp gene is needed to exhibit the rumplessness phenotype. The homozygous dominant condition (Rp/Rp) has been noted as a lethal gene, leading to embryonic death. Therefore, only heterozygous (Rp/rp+) or wild type (rp+/rp+) Araucana chickens survive, with only heterozygotes displaying the rumpless phenotype.

A differing genetic explanation for rumplessness can be seen in the Piao chicken. In this case, the mutation responsible for rumplessness is often attributed to an autosomal recessive mutation, though there has been some pushback on this conclusion. It is thought to be linked to a different gene or set of genes compared to the Araucanas, which explains how the mutation varies between species. Unlike the dominant mutation seen in the Araucanas, Piao chickens require two copies of the mutated gene (homozygous) to express the rumpless phenotype. Chickens with only one copy of the rumpless gene (heterozygous) will have normal tails and will carry the gene to pass on to offspring.

IRX1

The mutation that has been found to be the most directly responsible for causing rumplessness in chickens is one that occurs in the IRX1 (Iroquois homeobox 1) gene. IRX1 plays a critical role in the development of the caudal vertebrae and posterior somites during embryogenesis. Mutations in IRX1 lead to the absence or truncation of the tail in chickens, causing the rumpless phenotype. This mutation is seen in breeds like the Araucana chicken, contributing to the dominant autosomal nature of their mutation. Although a mutation in the IRX1 gene is most directly responsible for rumplessness, it likely interacts with other transcription factors, such as Hox genes, and signaling pathways in the developmental process of tail formation. Both of these gene families regulate vertebral column development, and mutations in one may influence the expression or function of the other.

A mutation in IRX1 may cause either a loss of function of misexpression of the gene. These types of mutation can disrupt the normal signaling and differentiation of the caudal somites in the embryo, resulting in impaired tail formation.

• Loss of function mutation: A loss of function mutation occurs when the mutation present on a gene causes the gene's function to be partially inhibited or completely nonfunctional. In this case, a loss of function mutation on the IRX1 gene would result in the caudal somites failing to differentiate properly. As a result of this, tail vertebrae will not form or only partially form, which leads to the rumpless phenotype seen.
• Misexpression: Misexpression of a gene occurs when the mutation causes the gene to be expressed in the wrong location or at the wrong time. In this instance, a misexpression of the IRX1 gene disrupts the proper patterning of the tail region, which leads to either defective tail formation or the absence of the tail vertebrae altogether.
  • Misexpression includes gain of function mutations, which have been noted in rumplessness of Araucana chickens.

One study in Piao chickens utilized signature analysis and structural variation detection to locate a 4174 base pair deletion in the upstream portion of IRX1 gene on chromosome 2. This deletion was found in all rumpless Piao chickens, and was absent in all tailed chickens in the study, showing a complete correlation between this deletion in IRX1 and rumplessness in Piao chickens.

Transcription factors and signaling pathways

Though mutations in the IRX1 gene have been noted to be the prime suspect in causing rumplessness, IRX1 interacts with numerous transcription factors and signaling pathways in embryonic tail development and differentiation to produce the tail of normal birds. Among these are the Hox genes, Fgf, and Wnt. Mutations in IRX1 therefore will interrupt these interactions as well, which contributes to rumplessness in chickens.

Hox genes

The Hox genes, specifically Hox10 and Hox11, which are responsible for specifying vertebral identity along with the body axis, have been found to function in tail development of chickens. These genes are involved in vertebral patterning and tail segmentation through somite development and specify the identity of each individual vertebrae along the anterior-posterior axis. Mutations in these genes generally result in structural defects in the tail, but have not been found to directly cause rumplessness.

Fgf and Wnt/β-catenin

In addition to Hox genes, IRX1 also interacts with signaling pathways, such as Fibroblast growth factor (Fgf) and the Wnt signaling pathway. Fgf is crucial to tail formation, specifically in the posterior portion of the embyro, and Wnt/β-catenin are significant in the regulation of somite differentiation and vertebral column development. IRX1 interacts with and may influence these pathways to help ensure the proper differentiation of somites into tail vertebrae. If these transcription factors and signaling pathways are disrupted by mutations in IRX1, normal tail development cannot occur, leading to the absence or incorrect development of the tail.

Developmental process summary

1. During the early stages of embryonic development, somites form in a segmented pattern along the anterior-posterior axis of the chicken embryo.
2. The IRX1 gene regulates the proper formation of somites at the posterior end of the embryo. These somites eventually give rise to the tail vertebrae.
3. A mutation in the IRX1 gene (either loss of function or misexpression) disrupts the normal differentiation of the caudal somites.
4. This disruption results in the absence or truncation of the tail vertebrae. As a result, the chicken may be rumpless or have a shortened tail.
5. The rumplessness phenotype is characterized by the lack of normal tail structures.

Breeding and conservation

Due to the unique rumpless characteristic of these breeds, breeding and conservation strategies have been implemented. These practices can allow these breeds to continue to thrive in their native habitats while attempting to reduce the negative effects that could be associated with rumplessness. For example, in breeds such as the Araucana, where the rumpless allele is dominant autosomal, heterozygous breeding is preferred. This practice will avoid potential embryonic lethality in homozygous dominant (Rp/Rp) individuals. Breeding strategies for rumpless chickens include introgression of the rumpless gene, breeding for genetic diversity, selection for breeding stock, and pedigree management.

• Introgression of the rumpless gene: For breeds like the Araucana, the goal is often to maintain rumplessness while also introducing desirable traits like egg color and feathering. This can be achieved through introgressive breeding, where the rumpless gene is repeatedly crossed into a target breed while selecting for desirable traits. Introgressive breeding is a fairly common practice for breeders, and has been used in the past for chickens, such as the Guizhou.
• Breeding for genetic diversity: On the other hand, rumplessness may not be the goal in some populations. In small, closed breeding populations like those used for conservation, inbreeding depression due to a lack of genetic diversity can lead to the accumulation of undesirable phenotypes that affect the health of the population.
• Selection for breeding stock: When breeding for rumplessness, it is important to select healthy, heterozygous individuals to breed. For the breeds for which rumplessness (Rp) is dominant, crosses between two heterozygotes will yield offspring that have a 25% change of being homozygous dominant (Rp/Rp, with the potential for severe defects), a 50% chance of being heterozygous (Rp/rp+, and therefore rumpless), and a 25% chance of being homozygous recessive (rp+/rp+, normal).
• Pedigree management: Keeping records of each bird's family history can help to manage genetic diversity and ensure that breeders are not selecting from a narrow gene pool. This can be accomplished through establishing a genetic studbook or breeding database to track the genetic lineage of each bird if possible.

Challenges in conservation

• Genetic bottlenecks: Many chicken breeds have faced genetic bottlenecks due to historical population declines or the loss of interest in their breed. This has led to a loss of genetic diversity, which can increase of health problems, and poses a risk to rumpless populations as well.
• Health and vitality: The homozygous condition of rumplessness (two copies of the IRX1 mutation) can be lethal, so conservation efforts need to focus on maintaining heterozygosity in the population to ensure that the birds can survive and reproduce. Breeding programs need to avoid the potential dangers of overrepresenting the rumpless gene pool.
• Preserving unique traits: For populations that are valued for traits other than rumplessness, breeding efforts may be shifted to produce offspring that display those traits instead. Ensuring that these characteristics remain in the population while maintaining health and genetic diversity can be a very delicate balance for breeders.

Advances in genetic testing now allow for the identification of specific alleles responsible for rumplessness. Molecular markers that identify the presence of the IRX1 mutation can be used to test breeding stock, ensuring that individuals with desirable genetic traits are selected for breeding.

In some regions, particularly those associated with Araucana and Manx Rumpy chickens, there are local conservation efforts to preserve rumpless chickens in small-scale, free-range environments. This allows the populations to maintain genetic diversity while being raised in a more natural setting. Furthermore, raising awareness about the importance of preserving rumpless breeds is key to their conservation. This includes educating breeders, farmers, and the public about the genetic challenges of and best breeding practices for preserving rumpless chickens.