Almost all lizard families in the pleurodont clade share the same XY system. This system was meticulously studied in Anolis carolinensis, where it shows a highly degenerated Y chromosome and a male-specific X chromosome dosage compensation mechanism. Corytophanids (casque-headed lizards) have been proposed as the only family in the pleurodont clade to lack the XY system. In this study, we worked with extensive genomic and transcriptomic data from Basiliscus vittatus, a member of the Corytophanidae family that inhabits the tropical rainforests of Mexico. We confirmed that B. vittatus underwent a sex chromosome system turnover, which consisted in the loss of the pleurodont XY system and the gain of a new pair of XY chromosomes that are orthologous to chicken chromosome 17. We estimated the origin of the sex chromosome system to have occurred ∼63 Ma in the ancestor of corytophanids. Moreover, we identified 12 XY gametologues with particular attributes, such as functions related to the membrane and intracellular trafficking, very low expression levels, blood specificity, and incomplete dosage compensation in males.

Clutch size (CS) and relative clutch mass (RCM) are considered important features in life history descriptions of species within Squamata. Variations in these two characteristics are caused by both biotic and abiotic factors. The present study provides the first account related to CS and RCM ofBasiliscus vittatusin Mexico within a population that inhabits an open riverbed juxtapositioned to tropical rainforest habitat in Catemaco, Veracruz, Mexico (170 m a.s.l.). Twenty-nine gravid females were collected and kept in captivity under favorable conditions that promote oviposition. The CS within this population was 6.2 ± 0.2 and was correlated positively with snout vent-length (SVL); while the RCM was 0.17 ± 0.006 and was correlated positively with both CS and width of egg. Factors, such as female morphology and environmental conditions, should influence these reproductive traits inB. vittatus. The data collected in this study could provide a framework for comparisons of the life history traits across populations ofB. vittatusin Mexico and within other species of the family Corytophanidae and provide a model for testing how abiotic and biotic factors may influence the CS and RCM in basilisk lizards throughout their range.

Resumen El tamaño y masa relativa de la camada y las particularidades de las crías son características importantes de historia de vida. El presente estudio aporta información sobre el tamaño y masa relativa de la camada y características de las crías de Phrynosoma orbiculare en el Parque Estatal Sierra Morelos, Toluca, Estado de México (± 2,700 m snm). Las hembras gestantes tuvieron una LHC de 75.2 ± 2.3 mm (n = 14), con un tamaño de camada de 8.3 ± 0.6 crías y una masa relativa de la camada de 27.2 ± 1.5 g. Las hembras de mayor talla y peso tuvieron un mayor número de crías, una situación común en lagartijas de ambientes templados; sin embargo, las hembras más ligeras tuvieron una mayor masa relativa de la camada que hembras de mayor peso; ambos resultados han sido registrados en lagartijas, incluyendo especies del género Phrynosoma. Las crías tuvieron una LHC de 26.1 ± 0.1 mm y un peso de 1.1 ± 0.01 g (sin diferencia sexual) y existe una variación baja en ambas características; la proporción sexual fue 1:1.

ResumenEn lagartijas ovíparas existe un intervalo frecuente en el desarrollo embrionario que alcanzan los huevos al momento de la oviposición entre el estadio 26 y 33. Dicha variación se ha relacionado con el origen de la viviparidad y permite la exploración de los efectos experimentales en el desarrollo embrionario después de la oviposición. Sin embargo, para la mayoría de las especies de lagartijas, el estadio embrionario en la oviposición es desconocido. El presente estudio detalla por primera vez en Corytophanidae el estadio embrionario en la oviposición de Basiliscus vittatus. Se colectaron hembras grávidas para obtener nidadas de reciente oviposición. De cada nidada, un huevo al azar fue disecado con la finalidad de determinar el estadio embrionario en la oviposición, además de medir variables morfométricas de la hembra y de los huevos. Los resultados muestran que en B. vittatus, las hembras ovipositan en 4 estadios embrionarios: 29 a 32; sin embargo, existe una preferencia por ovipositar en estadios embrionarios intermedios (estadios 30 y 31). De las diferentes variables registradas, el estadio en la oviposición solo se asocia con el largo máximo del huevo. El estadio en la oviposición de B. vittatus parece estar relacionado con factores fisiológicos y morfológicos de las hembras, así como por su estrategia de forrajeo.

Non-avian reptiles, unlike mammals and birds, have undergone numerous sex determination changes. Casque-Headed Lizards have replaced the ancestral XY system shared across pleurodonts with a new pair of XY chromosomes. However, the evolutionary forces that triggered this transition have remained unclear. An interesting hypothesis suggests that species with intermediate states, with sex chromosomes but also thermal-induced sex reversal at specific incubation temperatures, could be more susceptible to sex determination turnovers. We contrasted genotypic data (presence/absence of the Y chromosome) against the histology of gonads of embryos from stages 35–37 incubated at various temperatures, including typical male-producing (26°C) and female-producing (32°C) temperatures. Our work apparently reports for the first time the histology of gonads, including morphological changes, from stages 35–37 of development in the family Corytophanidae. We also observed that all embryos developed hemipenes, suggesting sex-linked developmental heterochrony. We observed perfect concordance between genotype and phenotype at all temperatures. However, analysis of transcriptomic data from embryos incubated at 26°C and 32°C identified transcript variants of the chromatin modifiers JARID2 and KDM6B that have been linked to temperature-dependent sex determination in other reptiles. Our work tested the validity of a mixed sex determination system in the family Corytophanidae. We found that XY chromosomes are dominant; however, our work supports the hypothesis of a conserved transcriptional response to incubation temperatures across non-avian reptiles that could be a reminiscence of an ancestral sex determination system.