Sex Chromosomes and Karyotype of the (Nearly) Mythical Creature, the Gila Monster, Heloderma suspectum (Squamata: Helodermatidae)

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.

Sex determination, longevity, and the birth and death of reptilian species

“…An initial reptilian SDM database was obtained from (The Tree of Sex Consortium, 2014) and complemented with an extensive literature search (Ota et al 1992;Gamble 2010;Pokorn a et al 2011Pokorn a et al , 2014Trifonov et al 2011;Badenhorst et al 2013;Matsubara et al 2013Matsubara et al , 2014Gamble et al 2014Gamble et al , 2015Koubov a et al 2014;Pokorna et al 2014;Rovatsos et al 2014a,b;Schmid et al 2014;Sulandari et al 2014). The resulting dataset contains information for 87 turtle and 303 lizard species (Table S1a) that have been studied across families (Table S1b).…”

Isolating Chromosomes of the Komodo Dragon: New Tools for Comparative Mapping and Sequence Assembly

“…Hybridization of our V. komodoensis paints onto metaphase chromosomes of other varanids may reveal the conservation of homologous syntenic blocks and the presence of intra-or interchromosomal rearrangements. It may also be interesting to hybridize V. komodoensis paints to metaphases of varanid outgroups such as the Gila monster ( Heloderma suspectum ), whose karyo- Pokorná et al [2014]. Interestingly, the diploid number (2n = 36) and chromosome morphology in H. suspectum resembles the putative ancestral karyotype of Toxicofera, the squamate clade that includes the clades Serpentes (snakes), Anguimorpha, and Iguania.…”

Transitions in sex determination and sex chromosomes across vertebrate species

“…The Tree of Sex data set we used is the same as that in the original publication with the addition of recently discovered XY systems in two species ( Boa imperator , Python bivittatus ) of snakes (Gamble et al., ). However, other recent studies in squamates are not included in this data set (Gamble et al., ; Koubová et al., ; Pokorná, Rens, Rovatsos, & Kratochvíl, ; Pokorná, Rovatsos, & Kratochvíl, ; Rovatsos, Altmanová, Pokorná, & Kratochvíl, ; Rovatsos, Pokorná, Altmanová, & Kratochvíl, ; Rovatsos et al., ). In many cases, we had congeneric matches between species in the tree and species in the data set.…”