Using Sex-reversed Chickens To Identify Core Spermatogenic Regulatory Genes
Lead Research Organisation:
University of Kent
Department Name: Sch of Biosciences
Abstract
Over the past decades, due to the protein rich contents of poultry products such as meat and eggs, there has been an exponential increase in commercial chicken production, and it is projected to remain one of the most popular livestock products by volume over the next 10 years. To meet the increasing global demand for poultry products, the production of chickens needs to be constantly expanded, which is dependent on the fertility and reproductive capacity of the male and female (parental) flocks. Commercial poultry management depends on optimal sperm production of relatively few breeding males, and maintaining and enhancing male fertility is a critical factor for large scale production. Selective breeding and restrictive feeding programmes are needed to maintain the fertility and breeding performance of the breeding males. The characteristics that influence the quality of sperm are sperm count, mobility, morphology, and viability, and intact acrosomes. In this research, we aim to understand the molecular network associated with the production of viable gametes by studying spermatogenesis in chickens. In particular, we will make use of a unique model system we have developed to study sperm production in birds with non-typical sex chromosomes. The sex chromosomes are well known to contain master regulators of fertility in many species, and this novel approach will allow us to rapidly identify sex-linked regulators of spermatogenesis.
In birds, males have two copies of the Z chromosome and females have one copy each of the Z and W chromosomes. Primordial germ cells (PGCs) are germline-competent cells that give rise to the functional gametes of an animal. It was previously thought that PGCs could only develop into either sperm (if ZZ) or oocytes (if ZW). However, we have recently shown that avian PGCs are bipotent, and can become either sperm or oocyte based on cellular signalling by the surrounding cells within the gonad. When male PGCs (carrying ZZ) are transplanted into a sterile female host (ZW), the ZZ cells form oocytes. Similarly, when female PGCs (ZW) are transplanted into a sterile male host (ZZ), the ZW cells can form functional sperm. Surprisingly, however, after the Z and W chromosomes separate during sperm production only Z-bearing cells survive to contribute to the next generation. We will study these sex-reversed cells in comparison to normal ZZ spermatogenesis, to understand whether the loss of W-bearing sperm in this model system is due to the presence of the W chromosome or the absence of a second Z chromosome. In either case, the findings will highlight key genes that are crucial for regulating cell survival, cell death and fertilizing ability of developing sperm cells. These genes can then be used as targets for selection to improve fertility in roosters.
Additionally, we will gather data on the basic biology of spermatogenesis in birds, focusing on two areas that are poorly understood. Firstly, in mammals, the sex chromosomes become inactive (a process known as MSCI, meiotic sex chromosome inactivation) during the meiotic cell stage of spermatogenesis. In chickens, evidence for MSCI is equivocal due to the difficulty accessing oocytes at this stage of development. Our new model is unique in that it allows us to study MSCI during spermatogenesis, where germ cells are much more accessible. Our research will resolve this controversy and offer conclusive evidence for the presence or absence of a sex chromosome transcription silencing mechanism in birds. Secondly, evidence from mammals shows that after the X and Y have been partitioned into separate cells during sperm production, they continue to share gene products (mRNA and protein) across "bridges" between developing cells. This process has however never been studied outside a mammalian context. ZW spermatogenesis offers a unique comparative system to understand the mechanisms and principles underpinning transcript sharing between developing sperm cells.
In birds, males have two copies of the Z chromosome and females have one copy each of the Z and W chromosomes. Primordial germ cells (PGCs) are germline-competent cells that give rise to the functional gametes of an animal. It was previously thought that PGCs could only develop into either sperm (if ZZ) or oocytes (if ZW). However, we have recently shown that avian PGCs are bipotent, and can become either sperm or oocyte based on cellular signalling by the surrounding cells within the gonad. When male PGCs (carrying ZZ) are transplanted into a sterile female host (ZW), the ZZ cells form oocytes. Similarly, when female PGCs (ZW) are transplanted into a sterile male host (ZZ), the ZW cells can form functional sperm. Surprisingly, however, after the Z and W chromosomes separate during sperm production only Z-bearing cells survive to contribute to the next generation. We will study these sex-reversed cells in comparison to normal ZZ spermatogenesis, to understand whether the loss of W-bearing sperm in this model system is due to the presence of the W chromosome or the absence of a second Z chromosome. In either case, the findings will highlight key genes that are crucial for regulating cell survival, cell death and fertilizing ability of developing sperm cells. These genes can then be used as targets for selection to improve fertility in roosters.
Additionally, we will gather data on the basic biology of spermatogenesis in birds, focusing on two areas that are poorly understood. Firstly, in mammals, the sex chromosomes become inactive (a process known as MSCI, meiotic sex chromosome inactivation) during the meiotic cell stage of spermatogenesis. In chickens, evidence for MSCI is equivocal due to the difficulty accessing oocytes at this stage of development. Our new model is unique in that it allows us to study MSCI during spermatogenesis, where germ cells are much more accessible. Our research will resolve this controversy and offer conclusive evidence for the presence or absence of a sex chromosome transcription silencing mechanism in birds. Secondly, evidence from mammals shows that after the X and Y have been partitioned into separate cells during sperm production, they continue to share gene products (mRNA and protein) across "bridges" between developing cells. This process has however never been studied outside a mammalian context. ZW spermatogenesis offers a unique comparative system to understand the mechanisms and principles underpinning transcript sharing between developing sperm cells.
Technical Summary
Chickens are the most abundant livestock animal because of their efficient growth and food conversion, cultural acceptability as a food source, and nutritional products, which are a vital source of protein for humans. Every year, on average, 73 billion chickens are slaughtered for their meat. Researchers are implementing innovative technology to enhance production of poultry products in response to this increasing demand. This project will study a unique sex-reversed model of spermatogenesis in order to identify sex-linked regulators of avian fertility. Primordial germ cells (PGCs) are germline stem cells that are widely used for biobanking chicken breeds and to produce genetically modified birds. In birds, males are homogametic (ZZ sex chromosomes) while females are heterogametic (ZW). Early stage PGCs have a sex specific transcriptome and proteome i.e. ZZ and ZW cells are intrinsically different. Despite this, however, we have shown that avian PGCs are bipotent, and in transplant experiments can be induced to form either sperm or oocytes dependent on the sex of the host embryo. In particular, female PGCs (ZW) survive through meiosis and divide to form sperm when transplanted into a sterile male host, but only the Z-bearing sperm survive. This offers a unique opportunity to use sex-reversed ZW spermatogenesis as a model system to identify master Z-borne regulators of fertility, while also addressing fundamental biological questions. In birds, meiotic sex chromosome silencing (MSCI) is a debatable topic where evidence is equivocal due to the difficulty in accessing chicken oocytes. ZW spermatogenesis provides a unique platform to investigate MSCI in a more accessible system. We hypothesize that MSCI is either incomplete or absent in birds; otherwise silencing of the Z would lead to spermatocyte death during or prior to meiosis. Instead, we believe W-bearing sperm die due to incomplete sharing of Z- and W-borne transcripts between sister spermatids following meiosis.
