Sexual Reproduction in Schmidtea mediterranea

By Anum Gaffar

What are the molecular markers used to characterize the hermaphroditic reproductive system of the planarian?

Modes of Reproduction

Schmidtea mediterranea have two distinct reproductive strains, a sexual and an asexual strain. The sexual strain develops into a hermaphrodite with reproductive organs that develop post embryonically. Whereas the asexual strain reproduces through transverse fission, without developing any reproductive organs. Further insight into the planarian can be found at the GeoChemBio website link.

Figure 1 Anatomy of the planiaran. (GeoChemBio)

Sexual Strain Reproduction

The cross-fertilized hermaphrodite’s reproductive system consists

  • Both male and female gonads
    • Testes are distributed dorsolaterally along the entire animal
    • Ovaries are localized ventrally at the posterior region of the cephalic ganglia
  • Accessory reproductive organs
  • Copulatory apparatus
    • Gonopore (gn)
    • Seminal vesicles (sv)
    • Copulatory bursa
    • Bursal Canal (bc)
    • Penis papilla (pp)
    • various other glands

Figure 2 The reproductive system of Schmidtea mediterranea (Chong 2011).

Spermatogenesis:

  • Sperm undergo three mitotic divisions
    • To produce eight primary spermatocytes that enter meiosis
  • The sperm are released into sperm ducts that guide the sperm to the seminal vesicles

Mating:

  • When mating the sperm from one animal is transferred to the other animal’s bursa canal inside the copulatory apparatus
  • The sperm travel back to the bursa canal and into the oviducts
  • They collect inside the Tuba, a large portion of the oviducts next to the ovaries
  • Once mature oocytes leave the ovaries they are fertilized by the sperm inside the tuba
  • The now fertilized eggs travel back down to the oviduct and yolk cells are added to the egg
    • Yolk cells are added by the Yolk glands that line the oviduct
  • The embryo with the yolk cells is packaged into a single egg capsule
  • Glands around the genital atrium synthesize and deposit egg capsules

Asexual Strain Reproduction

Transverse Fission: Transverse segmentation of the body

  • The planarian stretches and becomes thinner until the fission process occurs in posterior 2-3 of the animal
  • Two fragments are generated
    • Each regenerates the missing tissue, producing two organisms
  • Stimulated by higher temperature and darkness

Reproductive System consists of:

  • Primordial germ cells-Fail to differentiate further
  • No reproductive organs

Previously Known Facts

  • Neuropeptide signaling is required for sexual planarians
    • Knockdown of npy-8 (neuropeptide gene) resulted in a loss of developing reproductive organs.

Experiment

To identity differences between the two strains Chong and his colleagues used two approaches:

  1. Microarray Analysis
    1. To identify genes that are expressed differently in sexual and asexual planarians
    2. This was followed by a whole mount in situ hybridization to identify the cell types in which these genes are expressed
    3. Figure 3
  2. Morphological Analysis using confocal microscopy
    1. To identify cell type markers
    2. Also used FISH to label components of sexual strain reproductive organs
    3. Figure 4

Figure3: Transcriptional analysis of sexual versus asexual planarians (Chong 2011).

Figure 4 Genes upregulated in the sexual planarian that are expressed in the testes (Chong 2011).

Conclusions

  • Around 822 genes were expressed at higher levels in the sexual planarian
    • 42% of these genes were involved in diverse processes and had conserved domains.
      • Many of these genes were involved in cytoskeleton, signal transduction, cell cycle control/cell division/ chromosome partitioning
      • Found shared homology of these genes to proteins of unknown functions in other organisms
  • 129 genes were expressed at higher levels in asexual planarians
    • 11% had conserved domains
    • Other 89% were novel

Whole mount in situ hybridization showed that:

  • Of the 122 genes that showed differential expression in the sexual planiarans
    • 82% (100) were expressed in the reproductive system
      • 96 of the genes were expressed in the testes
      • 4 expressed in accessory reproductive organs
  • Transcripts of two genes were detected in accessory reproductive organs
    • A similar gene to granulin was expressed in cells forming the sperm ducts and seminal vesicles
    • A  similar gene to tsp-1 (tetraspanin) was expressed in glands around the atrium (forms the egg capsule)

FISH Analysis found transcripts of genes expressed in the testes

All four were found in all male germ cell stages except fully mature spermatozoa

  1. msy4
    1. Is suggested to be apart of the somatic cells of the testes
  2. thmg-1
    1. Is suggested to be apart of the somatic cells of the testes
  3. pde
  4. plastin

Final Thoughts:

  • >800 genes are upregulated in the sexual planiaran
  • Through whole mount in situ hybridization Chong and his colleagues found markers of the planarian reproductive system
  • The ability to label both male and female reproductive systems and with the markers identified will enable future scientists to dissect pathways and mechanisms that are involved in sexual differentiation and development.
  • Future studies
    • Can be done on the ovaries of the planarian
    • Use RNAi to generate animals devoid of male germ cells
    • Explore the possibility of simultaneous hermaphrodites into distinct male or female sexes

Paper Review

While the paper provided much insight into the planarian’s sexual strain it did not provide sufficient knowledge about the asexual strain. The paper could have gone more in depth about the differences between the two strains, and or could have possibly encouraged or proposed experiments on how to experimentally find differences between the two.

Bibliography

1. Chong, T. et al. “Molecular Markers to Characterize the Hermaphroditic Reproductive System of the Planarian Schmidtea Mediterranea.” BMC developmental biology 11.1 (2011): 69. Print.

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