Today marks a significant milestone in reptile genomics with the publication of two groundbreaking studies that present the near-complete reference genomes of the central bearded dragon (Pogona vitticeps). This species, prevalent in central eastern Australia and popular as pets across Europe, Asia, and North America, exhibits a unique trait: its sex is determined not only by genetics but also by the temperature of its nest. This dual mechanism has made the bearded dragon a valuable model for studying sex determination, and recent advancements in genomics have pinpointed a potential master sex determination gene crucial for male differentiation.
The independent verification of these findings by two separate research groups, employing different methodologies, underscores the robustness of the discovery. The studies reveal that the bearded dragon’s sex determination system is influenced by both genetic and environmental factors, specifically temperature. Unlike most animals, where sex is determined solely by chromosomes, bearded dragons can undergo sex reversal from male to female at high incubation temperatures. This means a lizard with male chromosomes can develop into a reproductively functional female if the egg is incubated at a sufficiently warm temperature.
Unveiling the Bearded Dragon’s Genetic Blueprint
Similar to birds and many reptiles, the central bearded dragon has a ZZ/ZW sex chromosome system, where females possess dissimilar ZW chromosomes, and males have identical ZZ chromosomes. The complexity increases as ZZ males can transform into phenotypic females at high temperatures without the involvement of the W chromosome. Recent technological advancements, such as ultra-long nanopore sequencing, have enabled the creation of telomere-to-telomere (T2T) assemblies of the sex chromosomes. This allows researchers to identify non-recombining regions and narrow down potential sex-determining genes.
The first study, conducted by researchers from BGI, the Chinese Academy of Sciences, and Zhejiang University, utilized DNBSEQ short-reads combined with long-reads from the new CycloneSEQ nanopore sequencer. This marks the first animal genome published using this technology. The second genome was assembled by a team from the University of Canberra, with contributions from multiple Australian and international institutions, using PacBio HiFi, ONT ultralong reads, and Hi-C sequencing. These efforts enable a direct comparison between the ONT and CycloneSEQ technologies, each offering unique insights into the sex determination process.
Key Genetic Discoveries
The sequencing of a ZZ male dragon by the BGI team focused on identifying genes specific to the Z chromosome, leading to the identification of Amh and Amhr2 (the Anti-Müllerian hormone gene and its receptor) as strong candidates for sex determination. Similarly, the Australian-led team, which sequenced a female ZW dragon, highlighted the same candidate genes. The expression studies revealed significant male-biased patterns for Amh, suggesting it as the master sex-determining gene. Furthermore, the differential expression of another gene, Nr5a1, suggests a more complex interaction, as it encodes a transcription factor with binding sites on the Amh promoter region.
“We anticipate accelerated research in other areas arising from these newly available assemblies,” said Arthur Georges from the University of Canberra.
Implications and Future Research
The discovery of genetic elements central to male sexual differentiation on the sex chromosomes is a major highlight of these assemblies. The genes Amh and Amhr2 have been duplicated onto the Z chromosome, making them prime candidates for the master sex-determining gene through a dosage-based mechanism. This finding fills a significant gap in our understanding of reptilian sex determination, as no master sex-determining gene akin to Sry in mammals or Dmrt1 in birds has been identified in reptiles until now.
Senior author Qiye Li from BGI expressed excitement over the complementary nature of the two reference genomes and their role in elucidating AMH signaling in sex determination. The research opens avenues for further exploration into the evolutionary origins of the ZW system and other aspects of vertebrate sex determination.
“The two reference genomes, derived from opposite sexes and generated by different technologies, are indeed complementary to each other,” said Qiye Li.
Next Steps and Broader Impact
The independent confirmation of these findings by two separate projects significantly enhances confidence in the results. The open sharing of data allows other researchers to build upon this work, particularly as some transcription factors linked to sex determination remain unresolved. The generation of these high-quality genome assemblies represents a substantial step forward in understanding sex determination in the bearded dragon.
A webinar with the lead authors is scheduled for August 26th at 10:00 am UTC, providing an opportunity for further discussion and questions. Interested parties can sign up to participate.
As the field of genomics continues to evolve, these studies not only advance our knowledge of reptilian biology but also pave the way for broader applications in comparative studies of sex determination across vertebrates.
