Teacher guide

Predict

In some species of lizards, snakes, fish and salamanders, every adult is female. There are zero males. Could a species with no males keep having babies for thousands of years?

Circle one. Yes / No

Watch

1. Some species are all female and they still make babiesyoutube.com/shorts/usPB5PGeb0E

Word of the day

Unisexual (yoo-nih-SEK-shoo-ul). Having only one sex. Every unisexual animal with a backbone that scientists have found is all female.

• A unicycle has one wheel.
• In music, voices singing in unison share a single sound. The word comes from Latin for “of a single sound.”

Questions

1. Whiptail lizards make babies by parthenogenesis, making babies without fertilization. In the video, Raven says whiptail babies are near copies of their mom. Why are whiptail babies near copies of their mom? And what’s one good thing about a species where every adult is female?

   Hint. Think about who adds DNA to a baby. Then think about how many adults in the species can lay eggs.

2. The Amazon molly is a fish. She needs sperm from a male of a different species to start her eggs. But her babies almost never get any of his DNA. Does the Amazon molly still count as an all-female species? Give your reason.

   Hint. Every Amazon molly is female, and her babies are almost always copies of their mom. How often does the male’s DNA end up in the babies?

3. Whiptails don’t need males. But sometimes two female whiptails act like they are mating. How could you test if mating-like behavior between two females helps whiptails have babies?

   Hint. Picture two groups of lizards. What would you change between the groups? What would you count?

4. The Brahminy blind snake spread around the world in the soil of potted plants. How does having no males help the Brahminy blind snake start a new group in a new place?

   Hint. If no male is needed, how many snakes does it take to start a new group?

Exit ticket

Predict

In some species of lizards, snakes, fish and salamanders, every adult is female. There are zero males. Could a species with no males keep having babies for thousands of years?

Circle one. Yes / No

Watch

1. Some species are all female and they still make babiesyoutube.com/shorts/usPB5PGeb0E

Word of the day

Unisexual (yoo-nih-SEK-shoo-ul). Having only one sex. Every unisexual animal with a backbone that scientists have found is all female.

• A unicycle has one wheel.
• In music, voices singing in unison share a single sound. The word comes from Latin for “of a single sound.”

Bonus word for question 2. Gynogenesis (jin-uh-JEN-uh-sis) means an egg needs sperm to start developing, but the sperm’s DNA almost never joins the egg’s DNA. It comes from gyno-, from the Greek for “woman,” and -genesis, “origin.”

Questions

1. In the video, Raven says parthenogenesis is making babies without fertilization, and the babies are near copies of their mom because there’s no dad adding DNA to the mix. A 2010 review by Kathrin Lampert and Manfred Schartl notes that mutations, small changes in DNA, still produce new genotypes in all-female species. Why does a baby made by parthenogenesis carry nearly the same DNA as her mom? And what can make her DNA a little different from her mom’s?

   Hint. Start with who adds DNA to the baby. Then think about what can change DNA from one generation to the next.

2. The Amazon molly needs sperm from a male of another species to start her eggs, but the sperm’s DNA almost never joins the egg’s DNA. A 2010 review by Kathrin Lampert and Manfred Schartl reports that some all-female salamanders also borrow sperm from males of a related species, and their babies sometimes keep part of the male’s DNA. What happens to the male’s DNA in a molly baby and in a salamander baby? And what evidence in a baby’s DNA would show that some of the male’s genes were kept?

   Hint. Start with whose DNA ends up in each baby. Then think about what you could compare a baby’s DNA against.

3. Whiptails don’t need males, yet two females sometimes act out mating behavior. Design a controlled experiment to test whether mating-like behavior between two females helps whiptails reproduce. Name the variable you change, the variable you measure, and a result that would show the behavior helps.

   Hint. Picture two groups of females that differ in one way only. Decide what you’d count in each group.

4. The Brahminy blind snake spread around the world in the soil of potted plants, and females can lay 1 to 8 eggs. Step by step, how could one Brahminy blind snake in one pot start a new population? And why would a two-sex snake species carried in the same pot be less likely to start a population?

   Hint. Follow one female and her eggs through two generations. Then think about who has to be in the pot for a two-sex species.

Claim, evidence, reasoning

Going further

1. A 2016 study by Newton and colleagues found that in New Mexico whiptail lizards, most egg cells stall partway through meiosis. The few egg cells that go on to become eggs start meiosis with twice the usual number of chromosomes, and identical copies pair up with each other. Why does pairing identical copies keep a whiptail baby’s gene mix the same as its mom’s? What does a two-parent baby get that neither a whiptail baby nor Coquita’s baby gets?

   Hint. Coquita’s baby lost almost all of her mixed gene pairs. Start with what each kind of baby gets from mom, then think about what a dad adds.

Exit ticket

Watch

1. Some species are all female and they still make babiesyoutube.com/shorts/usPB5PGeb0E

Word of the day

Unisexual (yoo-nih-SEK-shoo-ul). Having only one sex. Every unisexual animal with a backbone that scientists have found is all female.

• A unicycle has one wheel.
• In music, voices singing in unison share a single sound. The word comes from Latin for “of a single sound.”

Bonus word. Gynogenesis (from gyno-, “woman,” and -genesis, “origin”) describes development that sperm triggers without fusion of the sperm and egg nuclei. In the Amazon molly, the sperm’s DNA almost never joins the egg’s DNA.

Predict

1. In the paper below, David Crews and colleagues housed each female of an all-female whiptail, Cnemidophorus uniparens, with one partner of the same species. One kind of partner was intact. A second kind had her ovaries removed and doesn’t act out mating. A third kind had her ovaries removed and received progesterone, a hormone that makes her act out male mating behavior. Before you read, make a guess. Which partner led the most females to ovulate, and about what percent of females ovulated with each kind of partner? Write one sentence on why you think so.

   Hint. In the video, Raven says mating-like behavior helps trigger ovulation. Ask which partner acts most like a male.

Read the paper

Crews, Grassman and Lindzey, “Behavioral facilitation of reproduction in sexual and unisexual whiptail lizards,” PNAS, 1986. The paper is free to read as scanned pages.

David Crews’s team kept two whiptail species from Arizona in lab chambers that copied the light, temperature and humidity of southeastern Arizona in the breeding season. C. inornatus has males and females. C. uniparens is an all-female species that arose from hybrids, and C. inornatus supplied two-thirds of its triploid genome. Each C. inornatus female lived alone, with other females, with a castrated male, or with a castrated male given androgens, hormones that restore male mating behavior. Each C. uniparens female lived with a partner whose ovaries had been removed and who carried an empty capsule, with an intact partner, or with a partner whose ovaries had been removed and who carried a progesterone capsule. Females without ovaries don’t act out mating, but progesterone reliably triggers male-like mating behavior in them. Twice a week, the team felt each lizard’s belly to track her eggs. Table 1 counts the females that ovulated, releasing eggs from the ovary, during the study.

1. Quantitative reasoning Fill in the percent column. Add the two counts in a row to get the group total, divide the number ovulating by the total, then multiply by 100. What percent of C. uniparens females ovulated with each kind of partner, and about how many times higher was the percent with the progesterone-treated partner than with the partner given an empty capsule?

   Hint. Each group’s total is the number ovulating plus the number not ovulating.

2. Process of science Every C. uniparens partner in the study was a female of the same all-female species. Partners without ovaries got either an empty capsule or a progesterone capsule. Why did the team give one group of partners without ovaries an empty capsule, and what’s one other explanation for the progesterone result that the design leaves open?

   Hint. List everything the two kinds of partners share. Then ask what progesterone might change besides behavior.

3. Evolution In C. inornatus, the species with males, none of the 5 females kept alone ovulated, and 13 of 14 females housed with an androgen-treated male did. The authors write that the C. inornatus line is the maternal ancestor of C. uniparens. How do the two halves of Table 1 support the authors’ idea that the all-female C. uniparens kept a response to mating behavior from its sexual ancestor?

   Hint. Compare what raised ovulation in each species. Then ask who acted out mating in each case.

4. Evolution and systems In intact pairs of C. uniparens, the authors found that one female was past ovulation and acted male, while the other was about to ovulate and acted female. The surge in progesterone after ovulation triggers the male-like behavior, and the two females trade roles as their cycles move along. How do the two females’ hormone cycles let a pair take turns, and why would natural selection keep mating-like behavior in a species with no males?

   Hint. Follow one female through one cycle. Then compare the clutches of eggs laid by females alone and in pairs.

Exit ticket