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During fertilization, an egg and sperm cell fuse to create a new diploid structure. In humans, the process occurs once the egg has been released from the ovary, and travels into the fallopian tubes. The process requires several key steps: 1) sperm present in the genital tract must locate the egg; 2) once there, sperm need to release enzymes to help them burrow through the protective zona pellucida of the egg; and 3) the membranes of a single sperm cell and egg must fuse, with the sperm...
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During meiosis, chromosomes occasionally separate improperly. This occurs due to failure of homologous chromosome separation during meiosis I or failed sister chromatid separation during meiosis II. In some species, notably plants, nondisjunction can result in an organism with an entire additional set of chromosomes, which is called polyploidy. In humans, nondisjunction can occur during male or female gametogenesis and the resulting gametes possess one too many or one too few chromosomes.
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Spermatogenesis is the process by which haploid sperm cells are produced in the male testes. It starts with stem cells located close to the outer rim of seminiferous tubules. These spermatogonial stem cells divide asymmetrically to give rise to additional stem cells (meaning that these structures “self-renew”), as well as sperm progenitors, called spermatocytes. Importantly, this method of asymmetric mitotic division maintains a population of spermatogonial stem cells in the male...
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Mate choice—the decision about whom to mate with—is a type of natural selection, since animals must reproduce to pass down their genes. Mate choice is also called intersexual selection because the behavior occurs between the sexes.
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Most altruistic behavior—in which one animal helps another at a cost to themselves—occurs between relatives. Scientists think these altruistic behaviors evolved because they increase the inclusive fitness of the animal providing help.
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Updated: Feb 26, 2026

Assessing Differences in Sperm Competitive Ability in Drosophila
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Haploid selection within a single ejaculate increases offspring fitness.

Ghazal Alavioon1, Cosima Hotzy1, Khriezhanuo Nakhro1

  • 1Department of Evolutionary Biology, Uppsala University, SE-752 36 Uppsala, Sweden.

Proceedings of the National Academy of Sciences of the United States of America
|July 13, 2017
PubMed
Summary

In zebrafish, sperm variation within an ejaculate influences offspring fitness and survival. This study reveals genetic differences linked to sperm phenotype, impacting adaptive evolution in vertebrates.

Keywords:
biphasic life cyclegametic selectionsexual reproductionsperm genotypesperm selection

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Area of Science:

  • Evolutionary Biology
  • Genetics
  • Reproductive Biology

Background:

  • Eukaryotic sex involves alternating diploid and haploid phases, with haploid selection impacting adaptation and mutation load.
  • Haploid selection is established in plants, but animal sperm within an ejaculate are traditionally considered equivalent for offspring viability.

Purpose of the Study:

  • To investigate if phenotypic variation among intact, fertile sperm within a single ejaculate affects offspring fitness in vertebrates.
  • To explore the genetic basis of sperm phenotype variation and its consequences for offspring viability and fitness.

Main Methods:

  • Utilized zebrafish (Danio rerio) as a model organism.
  • Analyzed the impact of sperm longevity and motility on embryo survival, apoptosis, and adult offspring fitness.
  • Performed genetic analysis on sperm pools selected for different phenotypes.

Main Results:

  • Longer-lived sperm sired embryos with higher survival rates and reduced apoptosis.
  • Adult male offspring from selected sperm exhibited enhanced fitness.
  • Sperm pools selected by motility showed genetic differences across the genome.
  • The positive effects of sperm selection on embryo viability persisted into the second generation.

Conclusions:

  • Within-ejaculate sperm variation significantly impacts offspring fitness and survival in zebrafish.
  • Sperm phenotype is linked to genotype, with implications for vertebrate adaptive evolution.
  • Findings challenge the dogma of sperm equivalence within ejaculates and highlight the role of haploid selection in animals.