Chromosome Research

, Volume 21, Issue 2, pp 137–147 | Cite as

A ZZ/ZW microchromosome system in the spiny softshell turtle, Apalone spinifera, reveals an intriguing sex chromosome conservation in Trionychidae

  • Daleen Badenhorst
  • Roscoe Stanyon
  • Tag Engstrom
  • Nicole Valenzuela
Article

Abstract

Reptiles display a wide diversity of sex-determining mechanisms ranging from temperature-dependent sex determination (TSD) to genotypic sex determination (GSD) with either male (XY) or female (ZW) heterogamety. Despite this astounding variability, the origin, structure, and evolution of sex chromosomes remain poorly understood. In turtles, TSD is purportedly ancestral while GSD arose multiple times independently. Here we test whether independent (XY or ZW) or morphologically divergent heterogametic sex chromosome systems evolved in tryonichids (Cryptodira) using the GSD spiny softshell turtle, Apalone spinifera, a species with previously unidentified sex chromosomes. A female-specific signal from comparative genomic hybridization (CGH) was detected in a Giemsa/4′,6-diamidino-2-phenylindole faint portion of a microchromosome, indicating the presence of a ZZ/ZW system in A. spinifera. In situ hybridization of a fluorescently labeled 18S rRNA probe identified a large nucleolar organizer region block in the female-specific region of the W (co-localizing with the female-specific CGH signal) and a smaller block on the Z. The heteromorphic ZZ/ZW micro-sex chromosome system detected here is identical to that found in another tryonichid, the Chinese softshell turtle Pelodiscus sinensis, from which A. spinifera diverged ∼95 million years ago. These results reveal a striking sex chromosome conservation in tryonichids, compared to the divergent sex chromosome morphology observed among younger XX/XY systems in pleurodiran turtles. Our findings highlight the need to understand the drivers behind sex chromosome lability and conservation in different lineages and contribute to our knowledge of sex chromosome evolution in reptiles and vertebrates.

Keywords

Sex determination evolution reptiles sex chromosomes female heterogamety comparative genomic hybridization molecular cytogenetics vertebrates 

Abbreviations

2n

Diploid number

CGH

Comparative genomic hybridization

DAPI

4′,6-diamidino-2-phenylindole

FISH

Fluorescent in situ hybridization

gDNA

Genomic DNA

GSD

Genotypic sex determination

mya

Million years ago

my

Million years

NOR

Nucleolar organizer region

rRNA

Ribosomal RNA

TSD

Temperature-dependent sex determination

XX/XY

Male heterogamety

ZZ/ZW

Female heterogamety

Notes

Acknowledgments

We thank Jim Millard for providing the A. spinifera samples. Hawaiian P. sinensis samples were collected under Hawaii DLNR Department of Aquatic Resources Special Activities Permit 2011-061 with funding from the CSU Chico Center for Water and the Environment to T. Engstrom. This work was partially funded by NSF grant MCB 0815354 and supplements to N. Valenzuela and S.E. Edwards.

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Copyright information

© Springer Science+Business Media Dordrecht 2013

Authors and Affiliations

  • Daleen Badenhorst
    • 1
  • Roscoe Stanyon
    • 2
  • Tag Engstrom
    • 3
  • Nicole Valenzuela
    • 1
  1. 1.Department of Ecology, Evolution, and Organismal BiologyIowa State UniversityAmesUSA
  2. 2.Department of BiologyUniversity of FlorenceFlorenceItaly
  3. 3.Department of Biological SciencesCalifornia State UniversityChicoUSA

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