The strangest aspects of the human experience

Parthenogenesis

04/07/2022

Parthenogenesis is Greek for virgin birth. This is a form of asexual reproduction where embryos form without fertilization from sperm  

In sexual reproduction two cell types are required to create a diploid zygote, which is the earliest form of a fetus

  • Oocyte - egg cell
  • Spermatocyte - sperm cell

With parthenogenesis, that isn't the case

Episode: File 0066: Crime & Biology: Special Task Force Pt. 2

Release Date: April 8 2022

Researched and presented by Courtney

In parthenogenesis, the body finds a unique way of filling in for the genes usually provided by sperm.

Ovaries produce eggs through a complex process called meiosis, where the cells replicate, reorganize, and separate. These eggs contain only half the mother's chromosomes, with one copy of each chromosome. (These are called haploid cells; cells that contain two chromosomal copies are called diploid cells.)

The process of meiosis also creates a byproduct: smaller cells called polar bodies, distinct from the fertile egg. In one version of parthenogenesis called automixis, an animal can merge a polar body with an egg to produce offspring. This process, which has been documented in sharks, slightly shuffles the mother's genes to create offspring that are similar to the mother but not exact clones.

Meiosis - a process where a single cell divides twice to produce four cells containing half the original amount of genetic information

In another form of parthenogenesis, apomixis, reproductive cells replicate via mitosis, a process in which the cell duplicates to create two diploid cells-a kind of genetic copy-and-paste. Because these cells never undergo the gene-jumbling process of meiosis, offspring produced this way are clones of their parent, genetically identical. This form of parthenogenesis is more common in plants.

  • Mitosis - Mitosis is the process in which a eukaryotic cell nucleus splits in two, followed by division of the parent cell into two daughter cells. The word "mitosis" means "threads," and it refers to the threadlike appearance of chromosomes as the cell prepares to divide. 

Fitness - biological term referring the ability to survive to reproductive age, find a mate, and produce offspring. Basically, the more offspring an organism produces during its lifetime, the greater its biological fitness. This is what Darwin meant by "survival of the fittest" - not that the strongest survive like it's been taken to mean

Biological Sex

It's important to differentiate biological sex from gender. Gender is a social construct, an AFAB male, is "assigned female at birth, male" meaning biologically, their physical presentation and genetics aligned most closely to typical female presentation (or in archaic practices: individuals born intersex, this was the decision that was made by their doctors or parents), but that they would come to identify as male. 

Biological sex in humans is based on chromosomes. XX typically being female presenting and XY typically being male presenting. Right-wingers would have you think that biological sex is a binary thing, that you're one or the other, but things are rarely that clear cut. Biological sex is more of a spectrum, with female on one side and male on the other. Some humans are born leaning a lot more female biologically (due to hormones, chromosome presentation, DNA). But that doesn't mean that individual will grow up to identify as female. In humans, your odds of being born leaning either male or female is pretty even, the rare individual will fall somewhere closer to the middle of the spectrum (often referred to as intersex) and some individuals may even be born with two distinct sets of a genes, one more female leaning and one more male leaning, this is known as genetic chimerism. 

Sex Determination

How it works depends on the sex-determination system that that animal belongs to

  • Yeah, not all sexes are determined by XY like in humans. Sex is complicated. SO COMPLICATED.

  • Parenthogenesis is also found in humans *sometimes*

Sexual determination

  • Humans use XX/XY sex chromosomes

  • This is also true for most mammals

  • In this system most females have two of the same kind of sex chromosomes (XX) and most males have two distinct sex chromosomes (XY)

  • This means that the male haploid cell determines sex

  • Fruit flies also use XY sex chromosomes but sexual differentiation occurs AFTER fertilization

PLEASE NOTE that this NOT the only way a human (or other organism) can express sexual determination. This is literally a whole show on its own and super complicated and I will be researching in depth for another episode but for THIS episode I will not be going into more depth.

ALSO fuck TERFS, gender is a spectrum and a social construct, and Intersex bodies are valid and beautiful.

XX/X0 sex chromosomes

  • This is a variant of the XY system

  • Females have XX chromosomes but males only have one sex chromosome (X)

  • Sex is determined by the amount of genes expressed

  • This is observed in insects, although there are some mammals (mostly rodents) that also use a similar system, where both sexes lack a second sex chromosome but this isn't widely understood

ZW/ZZ sex determination

  • Females have ZW chromosomes, males have ZZ

  • Not all species depend on the W for sex, for example some butterflies express sex as ZW, but some females are Z0 or ZZW

This is all to say - sex determination is complicated, shit is wild, and nature is chaos.

Different kinds of Parthenogenesis

Arrhenotoky (Greek "birth of a male person")

  • Generally found in insects I.E. in bees

  • A haploid male is produced from an unfertilized egg

  • in bees this results in drone bees that can then mate with the female queen

Thelytoky (Greek for female birth)

  • Female organisms produced from unfertilized eggs

  • More common in inverts but also found in some vertebrates

  • Automixis is a form of thelytoky where meiosis takes place and then fusion occurs between nuclei in order to share genetic material between cells and create a "normal" diploid cell

  • This recombination of DNA doesn't occur as much/often as in sexual reproduction

  • This helps to prevent immediate inbreeding depression where the fitness of a population is decreased due to inbreeding

So why do this?
- sometimes it's more favorable to clone yourself instead of wasting resources on males who won't reproduce

  • Sometimes there aren't any males

  • In addition, sexual reproduction provides the benefit of meiotic recombination between non-sister chromosomes, a process associated with repair of DNA double-strand breaks and other DNA damages that may be induced by stressful conditions.

SO what animals use parthenogenesis ?

Common among a lot of different invertebrates, and found in birds as well.  But the poster child of this process is the whiptail lizard

The entire species is female! 

"Decades ago, behavioral neurobiologist David Crews read a strange report about the desert grassland whiptail, a small, slender lizard that lives in the sagebrush of the American Southwest. The paper claimed that the species was entirely female, and reproduced by cloning. It tested the limits of what Crews felt to be biologically plausible in higher vertebrates. "I didn't believe that such a thing existed," he says. But he was curious, and a friend who was going to New Mexico offered to collect some from the wild. Crews, who at the time was at the Harvard Museum of Comparative Zoology, installed a half-dozen whiptail lizards in glass tanks in his animal room. One day, he noticed a lizard biting at her cagemate's rear legs and tail, and soon after that, riding atop her. Crews instantly recognized that they were doing what lizards do when they have sex. But why would two females simulate the act of mating?"

To learn more about this, check out the Nautilus article: When Psuedosex is Better Than the Real Thing

Full Source List

Share