Volvox carteri
Scientific classification Edit this classification
(unranked): Viridiplantae
Division: Chlorophyta
Class: Chlorophyceae
Order: Chlamydomonadales
Family: Volvocaceae
Genus: Volvox
Species:
V. carteri
Binomial name
Volvox carteri
F.Stein 1878

Volvox carteri[1] is a species of colonial green algae in the order Volvocales.[2] The V. carteri life cycle includes a sexual phase and an asexual phase. V. carteri forms small spherical colonies, or coenobia, of 2000–6000 Chlamydomonas-type somatic cells and 12–16 large, potentially immortal reproductive cells called gonidia.[3] While vegetative, male and female colonies are indistinguishable;[4] however, in the sexual phase, females produce 35-45 eggs[4] and males produce up to 50 sperm packets with 64 or 128 sperm each.[5]

The genome of this species of algae was sequenced in 2010.[6] Volvox carteri is a significant model organism for research into the evolution of multicellularity and organismal complexity, largely due to its simple differentiation into two cell types, versatility in controlled laboratory environments, and natural abundance.[7]

Differentiation

Volvox carteri is a useful model organism for understanding the evolution and developmental genetics of cellular differentiation, in part because asexual colonies possess only two cell types. Approximately 2000 biflagellated somatic cells form a monolayer at the surface of the extracellular matrix (ECM) and cannot divide, rendering them mortal.[8] They facilitate motility in response to changes in light concentration (phototaxis), which is detected via an orange photoreceptor-containing eyespot.[8] Gonidia, by contrast, are immobile, embedded in the ECM interior, and are potentially immortal due to their ability to divide and participate in reproduction.[8]

Three key genes are known to play significant roles in the somatic-gonidium dichotomy: glsA (gonidialess A); regA (regenerator A); and lag (late gonidia). These genes are believed to carry out germ-soma differentiation during development in a general order:[9]

  1. gls specifies cell fate based on size
  2. lag genes facilitate gonidial development in large cells
  3. reg genes facilitate somatic development in small cells

The glsA gene contributes to asymmetric cell division that results in the designation of large cells that develop into gonidia and small cells that develop into somatic cells.[10] Gls mutants do not experience asymmetric division, a key component for creating gonidia, and thus are composed only of somatic swimming cells.[9]

The lag gene plays a role in specialization of gonidial initials.[9] If mutations disable the lag gene, large cells specified by glsA will develop as somatic cells initially but then de-differentiate to become gonidia.[11]

Determination of somatic cells is controlled by the transcription factor regA.[12] The regA geneencodes a single 80 amino acid-long DNA-binding SAND domain[13] that is expressed in somatic cells after embryonic development.[13][14] regA acts to prevent division by inhibiting cell growth via downregulation of chloroplast biosynthesis,[14] and represses expression of genes necessary for germ cell formation.[12] Chlamydomonas reinhardtii, a unicellular relative of V. carteri, is known to possess genes related to regA.[13] This suggests that the regA gene originated before proper cellular differentiation in Volvox and was likely present in an undifferentiated ancestor.[13] In this case, the function of regA in V. carteri most likely arose due to changes in expression pattern from a temporal (environmental response) state to a spatial (developmental) state.[15][16]

Genomics

The V. carteri genome consists of 138 million base pairs and contains c. 14,520 protein-coding genes.[6] Like many other multicellular organisms, this alga has a genome rich in introns;[6] approximately 82% of the genome is non-coding.[6] The V. carteri genome has a GC content of approximately 55.3%.[6][17]

Over 99% of the volume of a V. carteri colony is made up of a glycoprotein-rich extracellular matrix (ECM). Several genes involved in ECM construction and ECM proteins have been identified in V. carteri.[8] These genes account for the expanded inner layer of the cell wall (ECM) and the count and diversity of genes encoding VMPs (Volvox matrix metalloproteases) and pherophorins (ECM protein families).[6]

Volvox has multiple sex-specific and sex-regulated transcripts, including MAT3, an rb-homologous tumor suppressor that displays evidence of sex-specific selection and whose alternative splicing is sexually regulated.[17]

Sexual reproduction

V. carteri can reproduce both asexually or sexually. Thus, it is a facultatively sexual organism. In nature, Volvox reproduces asexually in temporary ponds in spring, but becomes sexual and produces dormant over-wintering zygotes before the ponds dry up in the summer heat. V. carteri can be induced to reproduce sexually by heat shock treatment.[18] However, this induction can be inhibited by antioxidants indicating that the induction of sex by heat shock is mediated by oxidative stress.[19] It was further found that an inhibitor of the mitochondrial electron transport chain that induces oxidative stress also induced sex in V. carteri.[20] It has been suggested that oxidative DNA damage caused by oxidative stress may be the underlying cause of the induction of sex in their experiments.[19][20] Other agents that cause DNA damage (i.e. glutaraldehyde, formaldehyde and UV) also induce sex in V. carteri.[21][22][23] These findings lend support to the general idea that a principal adaptive function of sex is repair of DNA damages.[24][25][26][27]

References

  1. ↑ Stein, Friedrich Ritter (1878). "I. Hälfte, Den noch nicht abgeschlossenen allgemeinen Theil nebst erklärung". Der Organismus der Infusionsthiere nach eigenen forschungen in systematischere Reihenfolge bearbeitet. III. Abtheilung. Die Naturgeschichte der Flagellaten oder Geisselinfusorien (PDF) (in German). Wilhelm Engelmann. p. 134.
  2. ↑ Guiry, M.D.; Guiry, G.M. "Volvox carteri". AlgaeBase. World-wide electronic publication, National University of Ireland, Galway.
  3. ↑ Lee, Robert Edward (2005) [1999]. Phycology (3rd ed.). Cambridge University Press.
  4. 1 2 Geng, Sa; Miyagi, Ayano; Umen, James (2018). "Evolutionary divergence of the sex-determining gene MID uncoupled from the transition to anisogamy in volvocine algae" (PDF). Development. 145 (7): dev162537. doi:10.1242/dev.162537. PMC 5963870. PMID 29549112.
  5. ↑ Herron, Matthew; Rashidi, Armin; Shelton, Deborah; Driscoll, William (2013). "Cellular differentiation and individuality in the 'minor' multicellular taxa: Differentiation and individuality". Biological Reviews. 88 (4): 844–861. doi:10.1111/brv.12031. PMC 4103886. PMID 23448295.
  6. 1 2 3 4 5 6 Prochnik SE, Umen J, Nedelcu AM, Hallmann A, Miller SM, Nishii I, Ferris P, Kuo A, et al. (2010). "Genomic analysis of organismal complexity in the multicellular green alga Volvox carteri". Science. 329 (5988): 223–226. Bibcode:2010Sci...329..223P. doi:10.1126/science.1188800. PMC 2993248. PMID 20616280.
  7. ↑ Kirk, David L. (1997). Volvox. Washington University in St. Louis: Cambridge University Press. pp. 13–15. ISBN 9780511529740.
  8. 1 2 3 4 Miller, Stephen M. (2010). "Volvox, Chlamydomonas, and the Evolution of Multicellularity". Nature Education. 3 (9): 65.
  9. 1 2 3 Wauchope, Akelia D. (2011). Advances in the molecular genetic analyses of Volvox carteri. UMT Dissertation Publishing. pp. 32–37.
  10. ↑ Kirk, David L.; Kaufman, MR; Keeling, RM; Stamer, KA (1991). "Genetic and cytological control of the asymmetric divisions that pattern the Volvox embryo". Dev. Suppl. 1: 67–82. PMID 1742501.
  11. ↑ Tam, L. W.; Stamer, K. A.; Kirk, D. L. (1991). "Early and late gene expression programs in developing somatic cells of Volvox carteri". Developmental Biology. 145 (1): 67–76. doi:10.1016/0012-1606(91)90213-M. PMID 2019325.
  12. 1 2 Herron, Matthew D. (2016). "Origins of multicellular complexity: Volvox and the volvocine algae" (PDF). Molecular Ecology. 25 (6): 1213–1223. doi:10.1111/mec.13551. PMC 5765864. PMID 26822195.
  13. 1 2 3 4 Hanschen, Erik R.; Ferris, Patrick J.; Michod, Richard E. (2014). "Early evolution of the genetic basis for soma in the volvocaceae". Evolution. 68 (7): 2014–2025. doi:10.1111/evo.12416. PMID 24689915. S2CID 10284100.
  14. 1 2 Meissner, M; Stark, K; Cresnar, B; Kirk, DL; Schmitt, R (1999). "Volvox germline-specific genes that are putative targets of RegA repression encode chloroplast proteins". Current Genetics. 36 (6): 363–370. doi:10.1007/s002940050511. PMID 10654090. S2CID 40307559.
  15. ↑ Herron, Matthew D.; Nedelcu, Aurora M. (2015). "Volvocine Algae: From Simple to Complex Multicellularity". Evolutionary Transitions to Multicellular Life. Advances in Marine Genomics. Vol. 2. pp. 129–152. doi:10.1007/978-94-017-9642-2_7. ISBN 978-94-017-9641-5.
  16. ↑ Nedelcu, Aurora M. (2009). "Comparative Genomics of Phylogenetically Diverse Unicellular Eukaryotes Provide New Insights into the Genetic Basis for the Evolution of the Programmed Cell Death Machinery". Journal of Molecular Evolution. 68 (3): 256–268. Bibcode:2009JMolE..68..256N. CiteSeerX 10.1.1.335.700. doi:10.1007/s00239-009-9201-1. PMID 19209377. S2CID 814557.
  17. 1 2 Ferris, P.; Olson, B. J.; De Hoff, P. L.; Douglass, S.; Casero, D.; Prochnik, S.; Geng, S.; Rai, R.; Grimwood, J. (2010). "Evolution of an expanded sex-determining locus in Volvox". Science. 328 (5976): 351–354. Bibcode:2010Sci...328..351F. doi:10.1126/science.1186222. PMC 2880461. PMID 20395508.
  18. ↑ Kirk DL, Kirk MM (January 1986). "Heat shock elicits production of sexual inducer in Volvox". Science. 231 (4733): 51–54. Bibcode:1986Sci...231...51K. doi:10.1126/science.3941891. PMID 3941891.
  19. 1 2 Nedelcu AM, Michod RE (November 2003). "Sex as a response to oxidative stress: the effect of antioxidants on sexual induction in a facultatively sexual lineage". Proceedings of the Royal Society B: Biological Sciences. 270 (Suppl 2): S136–9. doi:10.1098/rsbl.2003.0062. PMC 1809951. PMID 14667362.
  20. 1 2 Nedelcu AM, Marcu O, Michod RE (August 2004). "Sex as a response to oxidative stress: a twofold increase in cellular reactive oxygen species activates sex genes". Proceedings of the Royal Society B: Biological Sciences. 271 (1548): 1591–6. doi:10.1098/rspb.2004.2747. PMC 1691771. PMID 15306305.
  21. ↑ Starr RC, Jaenicke L (1988). "Sexual induction in Volvox carteri f. nagariensis by aldehydes". Sex Plant Reprod. 1: 28–31. doi:10.1007/bf00227019. S2CID 21915238.
  22. ↑ Loshon CA, Genest PC, Setlow B, Setlow P (July 1999). "Formaldehyde kills spores of Bacillus subtilis by DNA damage and small, acid-soluble spore proteins of the alpha/beta-type protect spores against this DNA damage". Journal of Applied Microbiology. 87 (1): 8–14. doi:10.1046/j.1365-2672.1999.00783.x. PMID 10432583.
  23. ↑ Zeiger E, Gollapudi B, Spencer P (March 2005). "Genetic toxicity and carcinogenicity studies of glutaraldehyde—a review". Mutation Research. 589 (2): 136–51. doi:10.1016/j.mrrev.2005.01.001. PMID 15795166.
  24. ↑ Bernstein H, Byerly HC, Hopf FA, Michod RE (September 1985). "Genetic damage, mutation, and the evolution of sex". Science. 229 (4719): 1277–1281. Bibcode:1985Sci...229.1277B. doi:10.1126/science.3898363. PMID 3898363.
  25. ↑ Birdsell JA, Wills C (2003). "The Evolutionary Origin and Maintenance of Sexual Recombination: A Review of Contemporary Models". In MacIntyre RJ, Clegg MT (eds.). Evolutionary Biology. Vol. 33. Springer. pp. 27–137. doi:10.1007/978-1-4757-5190-1_2. ISBN 978-0306472619.
  26. ↑ Hörandl, E. (December 2009). "A combinational theory for maintenance of sex". Heredity. 103 (6): 445–457. doi:10.1038/hdy.2009.85. PMC 2854797. PMID 19623209.
  27. ↑ Bernstein H, Bernstein C, Michod RE (2012). "Chapter 1: DNA repair as the primary adaptive function of sex in bacteria and eukaryotes". In Kimura S, Shimizu S (eds.). DNA Repair: New Research. Hauppauge, New York: Nova Science. pp. 1–49. ISBN 978-1-62100-808-8. Archived from the original on 29 October 2013.
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