Abstract
A wide variety of sex determination systems exist among squamate reptiles. They can therefore serve as an important model for studies of evolutionary transitions among particular sex determination systems. However, we still have only a limited knowledge of sex determination in certain important lineages of squamates. In this respect, one of the most understudied groups is the family Helodermatidae (Anguimorpha) encompassing the only two venomous species of lizards which are potentially lethal to human beings. We uncovered homomorphic ZZ/ZW sex chromosomes in the Gila monster (Heloderma suspectum) with a highly heterochromatic W chromosome. The sex chromosomes are morphologically similar to the ZZ/ZW sex chromosomes of monitor lizards (Varanidae). If the sex chromosomes of helodermatids and varanids are homologous, female heterogamety may be ancestral for the whole Anguimorpha group. Moreover, we found that the karyotype of the Gila monster consists of 2n = 36 chromosomes (14 larger metacentric chromosomes and 22 acrocentric microchromosomes). 2n = 36 is the widely distributed chromosomal number among squamates. In his pioneering works representing the only previous cytogenetic examination of the family Helodermatidae, Matthey reported the karyotype as 2n = 38 and suggested a different chromosomal morphology for this species. We believe that this was probably erroneously. We also discovered a strong accumulation of telomeric sequences on several pairs of microchromosomes in the Gila monster, which is a trait documented relatively rarely in vertebrates. These new data fill an important gap in our understanding of the sex determination and karyotype evolution of squamates.
Highlights
Squamate reptiles possess a notable variability in sex determination
One chromosome from the largest pair of microchromosomes was strongly heterochromatic in all females (Fig. 2c, d), but not in males (Fig. 2g, h), which identifies it as the W chromosome
We demonstrated that the Gila monster possesses homomorphic ZZ/ZW sex chromosomes with a highly heterochromatic W chromosome (Fig. 2)
Summary
Squamate reptiles possess a notable variability in sex determination. In this group, sex-determining mechanisms range from environmental sex determination, where the sex of an individual is decided by the environmental conditions present during the sensitive embryonic period and where there are no differences in genotypes between sexes, to genotypic sex determination with highly differentiated heteromorphic sex chromosomes [1,2]. Even basic information is scarce or even totally lacking for many important groups of squamates [2,3,13]. This precludes the more reliable phylogenetic analyses of sex determination in this group necessary for understanding the evolution of sex determination. In several popular or phylogenetically important lineages differential staining or methods of molecular cytogenetics often necessary for detection of sex chromosomes have not yet been applied
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