1. Sex and the Komodo Dragon
NATIONAL CENTER FOR CASE STUDY TEACHING IN SCIENCE
by Fiona Rawle1, Marc Dryer1, Joan Sharp2
1Department of Biology, University of Toronto at Mississauga
2Department of Biology, Simon Fraser University

2. What is this animal?

3. Background
The Komodo dragon, Varanus komodoensis, is the world’s largest lizard species.
Males are up to 3m in length, 90 kg in weight.
Females are up to 2m and 70 kg.

5. Komodo dragon resting

6. Young komodo dragon

7. Hind leg of a large komodo dragon

8. Young komodo dragon

9. Komodo dragons feeding on a Timor deer

12. How do Komodo dragons reproduce?

14. Prior to 2006, it was thought that Komodo dragons could only reproduce sexually.
However, in 2006, an interesting thing happened at two different British zoos that housed Komodo dragons…
Discuss egg laying here?

16. 2 female Komodo dragons (at different UK zoos) laid viable eggs without fertilization (i.e., no male sperm).
Flora: 11/25 (25 eggs laid, 11 were viable)
Sungai: 4/22 (22 eggs laid, 4 were viable)

17. How is it possible that a female Komodo dragon was able to produce offspring without any male genetic contribution?
Work as a group to come up with possible explanations. How could oogenesis be modified to produce diploid eggs that develop into a clutch of young dragons?

20. There are four possible explanations.

21. A diploid egg was produced by mitosis.

22. A haploid cell formed by meiosis I fused with the
first polar body to yield
a diploid egg.

23. A haploid cell formed by meiosis II fused with the
second polar body to yield
a diploid egg.

24. A haploid egg formed. The chromosomes in the egg underwent DNA replication. The sister chromatids separated during anaphase of mitosis, but then remained in a single cell as separate chromosomes to yield a diploid egg.

25. Genetic analysis of offspring
Consider the genetic makeup of the offspring based on each possible explanation, and answer the following questions:
Would the offspring be genetically identical to their mother (i.e., would they be a clone)?
Would the offspring be genetically identical to one another?
If the offspring are genetically different from one another and from their mother, how did the variation arise?
Would the offspring be homozygous/heterozygous at some/all loci?

26. CQ#1: If a diploid egg was produced by mitosis…
Select the correct statement:
The offspring would be genetically identical to their mother and to one another.
The offspring would not be genetically identical to their mother but would be genetically identical to one another.
The offspring and the mother would all be genetically different from one another.

27. CQ#2: If a diploid egg was produced by mitosis…
Select the correct statement:
The offspring would be homozygous at all genes.
The offspring would be heterozygous at some genes.

28. CQ#3: If a haploid cell formed by meiosis I fused with the first polar body to yield a diploid egg…
Select the correct statement:
Cells formed in this way would be genetically identical to their mother and to one another.
The resulting cells would not be genetically identical to their mother but would be genetically identical to one another.
Cells formed in this way and the mother would all be genetically different from one another.

29. CQ#4: If a haploid cell formed by meiosis I fused with the first polar body to yield a diploid egg…
Select the correct statement:
The resulting cell would be homozygous at all genes.
The resulting cell would be heterozygous at some genes.

30. CQ#5: If a haploid cell formed by meiosis II fused with the second polar body to yield a diploid egg…
Select the correct statement:
The offspring would be genetically identical to their mother and to one another.
The offspring would not be genetically identical to their mother but would be genetically identical to one another.
The offspring and the mother would all be genetically different from one another.

31. CQ#6: If a haploid cell formed by meiosis II fused with the second polar body to yield a diploid egg…
Select the correct statement:
The offspring would be homozygous at all genes.
The offspring would be heterozygous at some genes.

32. IF…
A haploid egg formed. The chromosomes in the egg underwent DNA replication. The sister chromatids separated during anaphase of mitosis but then remained in a single cell as separate chromosomes to yield a diploid egg.

33. CQ#7: Select the correct statement:
The offspring would be genetically identical to their mother and to one another.
The offspring would not be genetically identical to their mother but would be genetically identical to one another.
The offspring and the mother would all be genetically different from one another.

34. CQ#8: Select the correct statement:
The offspring would be homozygous at all genes.
The offspring would be heterozygous at some genes.

35. The DNA of the young dragons was analyzed.
All offspring were homozygous at all loci.
They were not identical clones of their mother: their genetic makeup was different from mum.
The offspring were not genetically identical to one other.
What does this tell us about the mechanism of parthenogenesis in Komodo dragons?

36. CQ#9: How did parthenogenesis occur in the Komodo dragon?
A diploid egg was produced by mitosis.
A haploid cell formed by meiosis I fused with the first polar body to yield a diploid egg.
A haploid cell formed by meiosis II fused with the second polar body to yield a diploid egg.
A haploid egg formed. The chromosomes in the egg underwent DNA replication. The sister chromatids separated during anaphase of mitosis but then remained in a single cell as separate chromosomes to yield a diploid egg.

37. CQ#10: What sex were the parthenogenetic offspring?
All offspring were male.
All offspring were female.
The offspring were half male, half female.

38. In the ZW system, ZW are females and ZZ are males.
While mammals have an XY system for sex determination, many reptiles (including Komodo dragons) have a ZW system for sex determination.
The Z chromosome is large and carries many genes (like the X chromosome of mammals).
The W chromosome is small, with few genes (like the Y chromosome of mammals).
In the ZW system, ZW are females and ZZ are males.

39. Birds also have ZW sex determination.
Karyotype of female red-winged blackbird: Karyotype of male oriole:

40. CQ#11: What sex were the parthenogenetic offspring?
All offspring were male.
All offspring were female.
The offspring were half male, half female.

41. Why were so many eggs non-viable? Discuss in your groups.
Many of the eggs produced by parthenogenesis did not develop to hatching.
Flora: Of 25 eggs laid, only 11 were viable.
Sungai: Of 22 eggs laid, only 4 were viable.
Why were so many eggs non-viable?
Discuss in your groups.

42. One of the female Komodo Dragons later reproduced sexually.
REPRODUCTIVE PLASTICITY
Female Komodo dragons may switch between asexual and sexual reproduction, depending on the availability of a mate.

43. What is the adaptive value of reproductive plasticity to a large (and thus scarce) lizard with ZW/ZZ sex determination that lives on a major archipelago?
Discuss in your groups how reproductive plasticity might be adaptive for Komodo dragons.

44. Image Credits
Slide 1: Varanus komodoensis by Dezidor, , CC BY 3.0.
Slide 2: Young Komodo dragon feeding at a water buffalo corpse on Rinca, by Mats Stafseng Einarsen, CC-BY-2.5.
Slide 4: Map showing Komodo dragon populations by Greg Holoboff.
Slides 3, 5-11, are all © Fletcher & Baylis, Wildside Photography, used in accordance with terms of use:
Slide 3, Slide 5: Komodo dragon resting,
Slide 6: Young Komodo dragon climbing tree to escape predation by larger dragons,
Slide 7: Hind leg of a large Komodo dragon,
Slide 8: Young Komodo dragon,
Slide 9: 4 Komodo dragons feeding on a Timor deer,
Slide 10: 2 Komodo dragons feeding on a Timor deer,
Slide 11: Komodo dragon feeding on a Timor deer,
Slides 13, 16, 35, 41, 42, 43: Komodo dragon hatchling, by Frank Peters, CC BY-NC-ND 2.0
Slide 15: Low resolution screenshot of BBC News page Wednesday, 20 December 2006, 18:03 GMT,
Slide 18: Diagram of oogenesis by Greg Holoboff.
Slide 19: Low resolution screenshot of Watts et al., 2006, “Parthenogenesis in Komodo dragons,” Nature 444:
Slide 39: Hobart HH et al Karyotypes of six species of North American Blackbirds (Icteridae: Passeriformes) The Auk. 99: