All-Female Species

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.

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