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In most organisms, sex is determined by the ratio of X and Y chromosomes. However, in some organisms, such as Drosophila and C.elegans, sex is determined by the ratio of the number of X chromosomes to the number of sets of autosomes. The Y chromosome in Drosophila is active but does not determine sex. It contains genes responsible for the production of sperms in adult flies.  
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In animals, gender is determined by the number and type of sex chromosome. For example, human females have two X chromosomes, and males have one X and one Y chromosome, whereas C.elegans with one X chromosome is a male, and the one with two X chromosomes is a hermaphrodite.
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The Y chromosome is a sex chromosome found in several vertebrates and mammals, including humans. In addition to 22 pairs of autosomes, the human males have one X chromosome and one Y chromosome. In these organisms, the presence or absence of the Y chromosome determines the development of male traits.
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Author Spotlight: Examining Volatile Sex Pheromone Influence on Male C. elegans Behavior
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Sex-limited experimental evolution drives transcriptomic divergence in a hermaphrodite.

Aivars Cīrulis1,2,3, Anna K Nordén1, Allison M Churcher4

  • 1Department of Biology, Lund University, 223 62 Lund, Sweden.

Genome Biology and Evolution
|December 29, 2023
PubMed
Summary

This study reveals that early evolution of sex chromosomes and distinct sexes involves significant changes in gene expression, particularly in testis-biased genes, during experimental evolution in Macrostomum lignano.

Keywords:
Macrostomum lignanoevolution of gonochorismexperimental evolutionhermaphroditesex chromosome evolutionsex-biased gene expressionsexual selection

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Area of Science:

  • Evolutionary biology
  • Genomics
  • Developmental biology

Background:

  • The transition from hermaphroditism to gonochorism involves sex chromosome formation and sex-biased gene expression.
  • Sexual selection is hypothesized to drive male-biased gene expression evolution.
  • Previous studies relied on theoretical models or ancient sex chromosomes.

Purpose of the Study:

  • Investigate gene expression changes during early sex chromosome evolution.
  • Examine adaptive gene expression under sex-limited experimental evolution.
  • Understand the role of sex-biased gene expression in the evolution of gonochorism.

Main Methods:

  • Experimental evolution of the simultaneous hermaphrodite Macrostomum lignano under three selection regimes: female-fitness, male-fitness, and control.
  • Characterization of whole-organism gene expression changes after 21-22 generations.
  • Gene ontology and Kyoto Encyclopedia of Genes and Genomes pathway analyses.

Main Results:

  • Female-selected lines exhibited the most substantial changes in gene expression.
  • Metabolic alterations, including amino acid biosynthesis and carbon metabolism, were identified as key adaptive components.
  • Testis-biased gene candidates showed a tendency towards downregulation in female-selected lines.

Conclusions:

  • Changes in testis-biased gene expression are crucial in the early stages of sex chromosome and gonochorism evolution.
  • Experimental evolution provides insights into the genetic underpinnings of sexual system transitions.
  • Metabolic adaptation plays a significant role in response to sex-limited selection pressures.