Gene Expression Dynamics during Temperature-Dependent Sex Determination in a Sea Turtle

Introduction

• Background: Study of temperature-dependent sex determination (TSD) in reptiles has been ongoing for over five decades. Specific incubation temperatures activate signaling pathways that influence embryo sex determination.

  • Charnov-Bull Model: TSD may have evolved due to differential fitness impacts on offspring ([Charnov and Bull, 1977]).
  • Example: Menidia menidia shows sexual dimorphism based on incubation temperatures.

• Turtles and TSD:

  • High temperatures (29.5-36°C) result in female offspring, low temperatures (20-26°C) in male offspring.
  • Focus on Aromatase (CYP19A1): Critical for estrogen synthesis in females.

• Recent Studies:

  • Gene KDM6B (or JMJD3) significant in TSD pathway for turtles like Trachemys scripta.
  • Mechanism: Acts as H3K27 demethylase and transcriptional activator.
  • Temperature Splicing: Temperature affects splicing of genes like KDM6B and JARID2.

• Olive Ridley Sea Turtle (Lepidochelys olivacea):

  • Shows TSD with thermosensitive stages 23-26.

Results

Transcriptome Analyses

• Developmental Stages and Tissues:

  • Focus on male (26°C) and female (33°C) producing temperatures.
  • Examined embryos and tissues: brain, liver, mesonephros, and gonads.

• Principal Component Analysis (PCA):

  • Distinct tissue clustering observed; gonad-specific genes showed clear temperature-based segregation.

Differential Expression Analysis

• Expression Profiles:

  • Comparison between 26°C and 33°C incubation temperatures.
  • Identified differentially expressed transcripts (DETs) across tissues.

• Key Findings:

  • Gonad-specific DETs showed significant expression increase from stage 23 to stages 25-26.
  • Temperature shifts affect global and specific gene expressions.

Temperature Shift Experiments

• Objective: Examine gene expression response to temperature changes.
• Findings:
  • Rapid and slow response genes identified.
  • Certain genes like JARID2 and KDM6B show rapid adaptation to temperature shifts.

Discussion

• Insights:

  • Temperature affects expression in multiple tissues, with a significant impact on gonads.
  • Key genes (AMH, DMRT1, CYP19A1) play roles in sex determination.

• Evolutionary Perspective:

  • Comparison with species having sex chromosomes suggests conserved genetic pathways.

• Future Questions:

  • Role of specific isoforms, regulatory factors in temperature-dependent splicing.

Materials and Methods

• Sample Collection: Eggs incubated at set temperatures, RNA sequenced from various tissues.
• Analyses: Transcriptome assembly, differential expression using EdgeR and statistical analyses performed.

Study Implications

• Broader Impacts: Findings advance understanding of genetic mechanisms in TSD.
• Research Opportunities: Explores evolutionary biology of sex determination and potential conservation strategies.