Coquita the Crocodile

Watch

1. Virgin birth in a crocodileyoutube.com/shorts/9Fil5Efiz9o
2. Coquita’s virgin birth is not a cloneyoutube.com/shorts/PkigBKfd-oU

Word of the day

Parthenogenesis (par-theh-no-JEN-uh-sis). An egg develops into an embryo without fertilization.

• The Parthenon in Athens took its name from Athena Parthenos, “Athena the Virgin.”
• You can spot the same gen root in gene and genetics. Gen means birth or origin.

Bonus words. Homozygous (from homo-, “same”) describes a matching pair of alleles. Heterozygous (from hetero-, “different”) describes a mixed pair.

Predict

1. A heterozygous site is a spot where Coquita carried two different versions of a DNA letter. A clone would keep 100% of those sites mixed. Before you read the paper, make a guess. What percent of Coquita’s heterozygous sites stayed heterozygous in her baby? Write one sentence on why you think so.

   Hint. In video 2, Raven says that at every spot where Coquita carried one working copy and one broken copy, her baby got two of the same.

Read the paper

Booth and others, “Discovery of facultative parthenogenesis in a new world crocodile,” Biology Letters, 2023. The paper is free to read.

Warren Booth’s team sequenced the genomes of Coquita and her baby. Table 1 of their paper counts the spots where Coquita carried two different versions of a DNA letter (her heterozygous sites) and how many of those spots stayed mixed in her baby. Each dataset uses sites spaced at least 10,000, 25,000 or 50,000 DNA letters apart.

1. Table 1 of the paper gives three datasets, with sites spaced 10,000, 25,000 or 50,000 DNA letters apart. What percent of Coquita’s mixed sites stayed mixed in each dataset, and why would Booth’s team run the same test at three different spacings?

   Hint. Fill in the last column of the table first. Then ask what could go wrong if many sites sat close together on a chromosome.

2. If Coquita’s egg had fused with the first polar body (central fusion), the baby would keep more than 33% of its mother’s mixed sites. If it fused with the second polar body (terminal fusion), it would keep less. Which mechanism do the Table 1 data support?

   Circle one. Central fusion / Terminal fusion

   Hint. Compare each percent you calculated with 33%.

3. All of the Table 1 data come from a single baby that grew in one egg. How much should the data from one baby convince scientists about how Coquita’s egg developed?

   Hint. Separate two questions. How sure can you be about this one egg, and how sure can you be about crocodiles in general?

4. Figure 2 of the paper maps the reptile lineages where facultative parthenogenesis has been documented, and the authors connect those lineages to birds. Did facultative parthenogenesis more likely evolve once in an ancestor shared by these reptiles and birds, or several times on its own? What evidence would tell the two apart?

   Hint. Think about how biologists decide whether a trait shared by related groups came from a common ancestor.

5. The authors suggest zoos test eggs from isolated females instead of throwing them away. What should a zoo that cares for a vulnerable species do with eggs from females kept alone? Write your recommendation in one paragraph.

   Hint. Think about what those eggs might hold and what the zoo would miss by discarding them.

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