9 - The Birds and the Bees, Eggs and Sperm

9. The Birds and the Bees, Eggs and Sperm

My earliest memories of life were whistling air and gentle clouds as the stork delivered me to my humble home: a wolfpack! As I grew older, I puzzled over the importance of reproduction and how genetic diversity played into the population-at-large around me. Today, we’ll discuss some of the reasons that DNA is important in acting as a blueprint; different ways that life has discovered to be able to reproduce; and some crazy folktales that have helped explain the birds and the bees. Let’s learn to be scientifically conversational.

 
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General Learning Concepts

1)     What is reproduction? What are gametes?

a.     What is reproduction? Something reproduced; a copy. This copying can be very accurate or very inaccurate. This gets into the differences between sexual or sexual reproduction; taking a portion of the parent body and using it for growth and differentiation in a new body.

b.     What are some important terms to know to understand reproduction?

i.     Chromosome: DNA (deoxyribonucleic acid, a genetic blueprint) that has been tightly coiled around a histone (protein, a molecular machine that acts as a scaffold) into a large, threadlike structure. Humans have 46 total chromosomes, 2 of which are sex chromosomes (either X or Y). All the DNA in the body (about 3 billion base pairs) is spread out across these chromosomes, in total numbering about 20,000 or so genes (the fundamental DNA sequence required to make some specific protein).

ii.     Haploid / diploid (ploidy): Ploidy refers to the number of sets of chromosomes. Haploid organisms only contain one set of chromosomes (23 chromosomes in humans), while diploids contain two sets (46 chromosomes). That means humans should have two copies of every gene. For example, I know that I have a mutation that makes me a carrier for cystic fibrosis. I do not exhibit the disease because I have a functioning gene that protects me, luckily, as a diploid. 

iii.     Gamete: An organism’s reproductive cells, or sex cells. In humans, female gametes are egg cells and male gametes are sperm cells. There is a process called meiosis that allows for a diploid parent cell to replicate its genome (4 copies of the chromosomes) and become four haploid cells.

iv.     Fertilization: When an egg and sperm (two haploid gametes) fuse they become a zygote (a newly diploid cell thanks to the fusion of two haploids). This process is known as fertilization.

v.     Gestation: The period between conception and birth; sometimes measured as the time it takes a fetus to fully develop and not the time from a zygote to a fetus. A human develops nine months; a mouse or rat for about 3 weeks (episode 4).

c.     Why is it important to have genetic variation?

i.     Genetic variation describes naturally occurring genetic differences among individuals of the same species. 

ii.     “If you can think of genetic value like you think of your retirement plan, a diversified portfolio minimizes risk and often provides the most reliable returns. Essentially, by having greater genetic diversity within a fish species (e.g., many discrete populations with different life history strategies rather than a single homogenized population), the species is more apt to withstand variable conditions.” – Abail Lynch, Research Fisheries Biologist at the National Climate Change and Wildlife Science Center.

iii.      Variation permits survival and flexibility of populations in a changing environment. This can be changed by nonrandom mating (mating based on certain traits; there would be higher changes of both genes containing cystic fibrosis if I exclusively tried to mate with carriers of the disease), genetic drift due to small populations (there are no individuals with straight hair), or distribution of individuals over a population (shiny fish in a different pond). [2]

d.     What is sexual reproduction vs. asexual reproduction? We’ve been speaking about sexual reproduction thus far; when two parents contribute genetic information to produce a unique offspring. Asexual reproduction is when reproduction creates a genetically identical offspring to a single parent.

2)     Examples of Sexual vs. Asexual Reproduction (and neither!):

a.     Sexual: Humans are a boring example: sea urchins are the next big thing! They live in tide pools / reefs. There are distinct sexes in sea urchins; poking a sea urchin in the correct place can allow for a sudden release of male gametes (sperm) that have several interesting techniques to arrive properly at the female sexed sea urchin instead of being diluted in the sea.

b.     Asexual: Bacteria (another domain of life, microns in size: if we were the size of a bacterium, the earth would be about as wide across as a bowling lane). Bacteria are traditionally thought to reproduce by binary fission; essentially growing to twice its size and splitting in two with all important machinery equally divided between the two daughter cells.

c.     Asexual in a vertebrate: Copperhead and cottonmouth vipers are capable of asexual reproduction. DNA sequencing shows “infinitesimally small” chances of broods sired by genetically similar snakes (to the mother). It is unclear why a snake would choose to give birth without a sexual reproductive step; some argue for and against separation with males. There are other vertebrates that do this, including other snakes, sharks, and lizards.

d.     Viral replication: Viruses do not technically reproduce because they’re incapable of doing it by themselves (this does not apply to parasites, which often reproduce inside of other organisms but are alive; this tends to be explained by not using typical cell division). Viruses must use the host’s machinery in order to turn their mRNA (ribonucleic acid, the message of DNA) into protein. Therefore, they tend to be known to replicate, not reproduce.

3)     Pharmaceuticals that disturb gestation or fertilization:

a.     Birth Control: A hormonal (chemical molecules that signal to your body to do specific things) technique that prevents a female gamete (egg) from being available to be fertilized by a male gamete (sperm). We’ll have an episode where we talk about birth control in more detail on its own.

b.     Thalidomide: A now-known teratogen (factor that causes malformation of an embryo) that was prescribed as a morning sickness pill around 60 years ago. Thalidomide resulted in severe birth defects in 10,000 or so children; upper limbs were disfigured while internal organs, genitalia, and spinal column showed signs of being affected. Severe cases had mortality rates of 40%. Thalidomide had two active states (chiral carbon, enantiomers coexist and the drug can racemise), with one being teratogenic. A racemic mixture was sold. It is unclear how the drug biochemistry caused embryopathy but proposals include DNA mutagenesis, inhibition of certain proteins, or toxicity to nerves.

4)     Fun Tidbits

a.     Sexy Male, Choosy Female: Ronald Fisher wrote in 1930 that female mating preferences are recognized for evolution of male sex characteristics. Choosy females select sexy males that would make their young “sexier” and easier for those offspring to mate in turn. In general, this really all boils down to sexual selection for organisms and methods that some organisms use to be more successful at mating. 

b.     Where does the phrase ‘the birds and the bees’ come from? Unclear initial origins. Bees pollinating flowers generally symbolize male fertilization while birds laying eggs equate to female ovulation. Altogether, a confusing metaphor.

i.     USC professor Ed Finegan found an earlier use of the phrase in the diary of John Evelyn, published in 1644 (but written a century prior): “That stupendous canopy of Corinthian brasse; it consists of 4 wreath'd columns--incircl'd with vines, on which hang little putti [cherubs], birds and bees.”

ii.     1928: Cole Porter's lyrics to the 1928 song "Let's Do It." Birds do it, bees do it / Even educated fleas do it / Let's do it, let's fall in love. This is one of the earlier references with implications to reproduction.

c.     Where does the stork come from? It is difficult to decode the history of any myth. There are legends of storks from Greek, Slavic, and German folklore. Specifically, some German folklore says that babies were found in caves or wetlands by storks and brought to households in a basket by storks. To indicate that a household wanted a child, the occupants would place a sweet for the stork on the windowsill. This legend was populatized by a 19th-centruy Hans Christen Andersen story called ‘The Storks’.

5)     Solicited Naïve Questions

a.     How can I become more successful with sexual reproduction? Often, higher order organisms will develop secondary sexual characteristics: these are body features other than reproductive organs that appear during maturation. Proposed by Charles Darwin, secondary sexual characteristics include the elaborate tails of peacocks, expandable throat sacks in birds, lack racks in moose, and so on. Because this question was asked by a fish, Xiphophorus helleri (a Heckel), I recommend allowing your pelvic fin to darken as you age. Because you’re totally translucent, it’s easily seen and a very attractive feature to your potential mates.

b.     Do praying mantises kill their partner after copulation? Animal cannibalism has been seen in a variety of different animals, including sharks, but generally not in a sexual context. Animals like mantises, spiders, and potentially scorpions can or will eat each other during mating. About ¼ of approaching mantid males are eaten by the female… or at least the head. Sexual cannibalism can be beneficial to the species by providing nutrients for a superior number of eggs or fertilized eggs.

 
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