1. Robin Wright wrightr@umn.edu
Primer on Parthenogenesis & some other interesting variations in vertebrate reproduction
Robin Wright

2. Some Variations in Vertebrate Reproduction
M1 M2
M1 M2
Meiosis or
Mitosis
Development
Parthenogenesis
egg
Gynogenesis
egg
M1 M2 M1 M2
P1 P2
P1 or P2
M1 M2
Paternal
genome
eliminated
Meiosis
Fertilization
Development
Hybridogenesis
egg
M1 P1 M1
or M1 P3
P3 P4
P3 or P4
M1 P3
or M1 P4 P2
Fertilization
Meiosis:
Paternal genome
eliminated
Development
Homologs
A b a B
M1 M2
(P1 P2)
(P3 P4)
Haplotype
This slide introduces several variations in reproduction.
Note the chromosome & genome terminology on the left
A diploid cell is shown, with 2 pairs of homologous chromosomes; each SET of chromosomes is a haplotype
For this diagram, M1 and M2 refer to maternal haplotypes and P1, P2, P3, and P4 refer to paternal haplotypes.
Homologs – homologous chromosomes
M1 – one chromosome set in the original female, therefore a set of alleles that tend to be inherited together; the M refers to Maternal origin
M2 – the other chromosome set, therefore a second set of alleles that tend to be inherited together; also of maternal origin here (although one set presumably came from a male at some time in the evolutionary history of the female)
P1 – a set of chromosomes alleles from the male
Some Variations in Vertebrate Reproduction
Parthenogenesis
eggs produced by mitosis or meiosis; undergo development without fertilization & with only genes derived from the mother
Mitosis: offspring are exact clones (except for mutations that occur)
Meiosis: because of crossing over, offspring are not exact copies; but are very similar to the mother; this mode of reproduction tends to preserve heterozygosity
Examples in snakes, lizards, fish
Clipart of lizard comes from Microsoft Office
Gynogenesis
Eggs produced by meiosis; because of crossing over, offspring are not exact copies; but are very similar to the mother; this mode of reproduction tends to preserve heterozygosity
Fertilized by males from sibling species
Male genome discarded prior to embryonic development
Examples in fish
Clipart from Microsoft Office
Hybridogenesis
Eggs produced by meiosis, during which the paternal genome is discarded
Because of crossing over and mutations, the eggs do not have exactly the same genome as the mother
Egg is fertilized by males; produce mostly heterozygous (hybrid) offspring
In this mode of reproduction, a male can be a genetic father, but not a genetic grandfather
Examples in Frogs & fish
Clipart of frog from

3. Production of Parthenogenetic Eggs
metaphase
metaphase
metaphase
metaphase
Mitosis
No change in chromosome number;
Egg is genetic clone of mother
Failure of Meiosis II
Meiosis I
Meiosis I
Meiosis II A B C D
Fusion of egg&
second polar body
Meiosis I
Meiosis II
Mitotic Duplication of
Chromosomes after Meiosis
DNA replication
DNA replication
Diploid Maternal
Cell
This slide compares how parthenogenetic eggs can be produced.
Crossing over is not illustrated in this slide, but is included in the next one.
Production of Parthenogenetic Eggs by Mitosis
Eggs have identical genome as mother (except for mutations that may arise)
All offspring are female clones of each other and of mother
Production of Eggs by Meiosis
Failure of Meiosis II
Various mechanisms: meiosis with only 1 cell division; fusion of first meiotic cells (ovum & first polar body)
Because of independent assortment, crossing over (and mutations) the offspring are not identical to the mother, but are very similar (half clones – progeny carry ½ of mother’s alleles)
Heterozygosity lost over time
Offspring may be male or female or either depending on sex determination system
If sex chromosomes are present, only the embryo with two copies of the large chromosome (i.e. homogametic) will be viable
Reptiles, amphibians, & birds - homogametic sex is usually male
Fish – homogametic sex is usually female
If sex is determined by a gene on an pair of indistinguishable homologous chromosomes, both viable males and females will be produced
Fusion of ovum and second polar body
Fusion of ovum and one of the cells produced by meiosis II (i.e. ovum & second polar body)
Because of independent assortment & crossing over (and mutations) the offspring are not identical to the mother, but are very similar
Tends to preserve heterozygosity
Offspring can be male or female or both, depending on sex determination mechanism (produces only heterogametic sex)
Reptiles, amphibians, & birds – heterogametic embryo is usually female
Fish – heterogametic embryo is usually male
Duplication of Chromosomes after Meiosis
One round of DNA replication at the end of meiosis
restores diploidy in haploid products of meiosis
Progeny are not identical to the mother, but are homozygous at each locus
Offspring can be male or female, depending on sex determination mechanism (produces only homogametic sex)
Duplication of Chromosomes before Meiosis (not shown)
One round of DNA replication before the beginning of meiosis, resulting in four copies of each chromosome (tetraploidy) at meiosis I
Progeny are exact clones of mother
Possible Egg Genotypes = Possible Genotypes of Parthenogenetic Offspring

4. Production of Parthenogenetic Eggs
metaphase
metaphase
metaphase
metaphase
Mitosis
No change in chromosome number;
Egg is genetic clone of mother
Failure of Meiosis II
Meiosis I
A+C
A+D
B+C
B+D
Meiosis I
Meiosis II A B C D
Fusion of egg&
second polar body
Meiosis I
Meiosis II
Mitotic Duplication of
Chromosomes after Meiosis
DNA replication
DNA replication
Diploid Maternal
Cell
This slide is the same as the previous, but includes crossing over; note that we start the diagram at metaphase, so crossing over has already happened; we include crossing over to illustrate that meiosis produces 4 chromosomes with unique allele combinations. So, the four offspring produced by parthenogenesis would not be identical, even though they carry only alleles that are present in the mother.
Production of Parthenogenetic Eggs by Mitosis
Eggs have identical genome as mother (except for mutations that may arise)
All offspring are female clones of each other and of mother
Production of Eggs by Meiosis
Failure of Meiosis II
Various mechanisms: meiosis with only 1 cell division; fusion of first meiotic cells (ovum & first polar body)
Because of independent assortment, crossing over (and mutations) the offspring are not identical to the mother, but are very similar (half clones – progeny carry ½ of mother’s alleles)
Heterozygosity lost over time
Offspring may be male or female or both depending on sex determination system
If sex chromosomes are present, only the embryo with two copies of the large chromosome (i.e. homogametic) will be viable
reptiles, amphibians, & birds - homogametic sex is usually male
Fish – homogametic sex is usually female
If sex is determined by a gene on an pair of indistinguishable homologous chromosomes, both viable males and females will be produced
Fusion of ovum and second polar body
Fusion of ovum and one of the cells produced by meiosis II (i.e. ovum & second polar body)
Because of independent assortment & crossing over (and mutations) the offspring are not identical to the mother, but are very similar
Tends to preserve heterozygosity
Offspring can be male or female or both, depending on sex determination mechanism (produces only heterogametic sex)
Reptiles, amphibians, & birds – heterogametic embryo is usually female
Fish – heterogametic embryo is usually male
Duplication of Chromosomes after Meiosis
One round of DNA replication at the end of meiosis
restores diploidy in haploid products of meiosis
Progeny are not identical to the mother, but are homozygous at each locus
Offspring can be male or female, depending on sex determination mechanism (produces only homogametic sex)
Duplication of Chromosomes before Meiosis (not shown)
One round of DNA replication before the beginning of meiosis, resulting in four copies of each chromosome (tetraploidy) at meiosis I
Progeny are exact clones of mother
Possible Egg Genotypes = Possible Genotypes of Parthenogenetic Offspring

5. What if parthenogenesis occurred in mammals?
X X
X X
X X
Failure of Meiosis II
Meiosis I
Diploid
Maternal
Cell
X X
DNA replication
This example is theoretical. Parthenogenesis has not been observed in mammals, probably because of imprinting requirements. Because female mammals are X X, only female offspring would be produced by parthenogenesis.
You may also want to go through meiosis in a male to illustrate that the sperm will carry either an X or a Y chromosome, but not both.
This slide shows a mammalian style XX meiosis to compare to the snake example shown in the next slide. This example is only included as a starting point – parthenogenesis does not occur in mammals because of genetic imprinting: to be properly expressed, some genes need to be inherited from the mother and others from the father. There are LABORATORY examples of parthenogenesis, but it involves a lot of manipulation (fusion of an egg nucleus with a nucleus from an immature egg.) In addition, one might argue that cloning in which an organism is produced by replacing an egg nucleus with the nucleus from another cell is also a type of “facilitated” parthenogenesis.

6. All male or all female offspring produced
Sex Determination & Parthenogenesis
Failure of Meiosis II
Meiosis I
Z W
ZZ WW ZZ
Meiosis I Z W ZW
Z W
Meiosis II
ZZ WW
Fusion of ovum &
second polar body
Meiosis I Z W ZZ
Z W
Meiosis II
ZZ WW
Mitotic Duplication of
Chromosomes after Meiosis
DNA replication
Z W
Diploid
Maternal
Cell
This slide does not include a mammalian style XX meiosis for comparison.
Note that when sex determination involves genes carried on sex chromosomes, parthenogenesis can produce all females or all males, depending on the specific sex determination mechanism and how the diploid egg is produced.
The examples show cells from an organism with a ZW genetic sex determination mechanism, which is common in birds, reptiles, and amphibians. (Sex determination varies a lot in reptiles, sometimes being dependent on temperature of incubation.) In most of species of birds & reptiles (but not all!), females are heterogametic (ZW) and males are homogametic (ZZ).
If the parthenogenetic mechanism involves failure of meiosis II, all viable progeny will be homozygous for the Z chromosome and, therefore, female. (The eggs that are ww can’t develop into a viable offspring.) This was what Prof. Chiszar observed. Typically, “failure of meiosis II” involves cell fusions rather than the absence of a second division. Typically, the ovum fused with the first polar body, to form a diploid egg that contains both copies of each sister chromatid.
In an alternative system, the egg can be produced by fusion of the ovum with a second polar body. This fusion would result in one copy of each chromosome in the progeny. That is, the egg would contain one sister chromatid from one duplicated chromosome and one sister chromatid from the HOMOLOGOUS duplicated chromosome. In this case all of the progeny would be heterozygous ZW, and therefore be male.
Finally, following meiosis, the chromosomes may be duplicated, thus producing eggs that are homozygous at all loci. Only the ZZ eggs can develop. The WW eggs are missing some essential genes.
Other systems of sex determination that involve genes on homologous chromosomes (rather than on sex chromosomes) could produce both males and females. This process is not shown, but easy to work out from the previous slide. Simply add a “sex gene” with one “male” allele to one of the homologous chromosomes and follow it through the various types of meiosis-based parthenogenetic egg formation.
All male or all female offspring produced

7. All male or all female offspring produced
Sex Determination & Parthenogenesis
Failure of Meiosis II
Meiosis I
Z W
ZZ WW ZZ
Meiosis I Z W ZW
Z W
Meiosis II
ZZ WW
Fusion of ovum &
second polar body
Meiosis I Z W ZZ
Z W
Meiosis II
ZZ WW
Mitotic Duplication of
Chromosomes after Meiosis
X X
X X
X X
Failure of Meiosis II
Meiosis I
DNA replication
Z W
Diploid
Maternal
Cell
This example includes what would result from parthenogenesis in mammals; however it does not occur because of genetic imprinting: to be properly expressed, some genes need to be inherited from the mother and others from the father. There are LABORATORY examples of parthenogenesis, but it involves a lot of manipulation (fusion of an egg nucleus with a nucleus from an immature egg.) In addition, one might argue that cloning in which an organism is produced by replacing an egg nucleus with the nucleus from another cell is also a type of “facilitated” parthenogenesis.
Note that when sex determination involves genes carried on non-homologous sex chromosomes, parthenogenesis can produce all females or all males, depending on the specific sex determination mechanism and how the diploid egg is produced.
The examples show cells from an organism with a ZW genetic sex determination mechanism, which is common in birds, reptiles, and amphibians. (Sex determination varies a lot in reptiles, sometimes being dependent on temperature of incubation.) In most of species of birds & reptiles (but not all!), females are heterogametic (ZW) and males are homogametic (ZZ).
If the parthenogenetic mechanism involves failure of meiosis II, all viable progeny will be homozygous for the Z chromosome and, therefore, female. (The eggs that are ww can’t develop into a viable offspring.) This was what Prof. Chiszar observed. Typically, “failure of meiosis II” involves cell fusions rather than the absence of a second division. Typically, the ovum fused with the first polar body, to form a diploid egg that contains both copies of each sister chromatid.
In an alternative system, the egg can be produced by fusion of the ovum with a second polar body. This fusion would result in one copy of each chromosome in the progeny. That is, the egg would contain one sister chromatid from one duplicated chromosome and one sister chromatid from the HOMOLOGOUS duplicated chromosome. In this case all of the progeny would be heterozygous ZW, and therefore be male.
Finally, following meiosis, the chromosomes may be duplicated, thus producing eggs that are homozygous at all loci. Only the ZZ eggs can develop. The WW eggs are missing some essential genes.
Other systems of sex determination that involve genes on homologous chromosomes (rather than on sex chromosomes) could produce both males and females. This process is not shown, but easy to work out from the previous slide. Simply add a “sex gene” with one “male” allele to one of the homologous chromosomes and follow it through the various types of meiosis-based parthenogenetic egg formation.
All male or all female offspring produced
Hypothetical mammal

8. Handout: Sex Determination in Parthenogenesis
X X
Meiosis I
Meiosis I
Z W
Meiosis I
Z W
Meiosis I
Z W
Diploid
Maternal
Cell
Z W
DNA replication
Potential Handout to use in class
Hypothetical mammal

9. Parthenogenesis in all Female Species
M1 M2
M1 M2
Clone of Mother
Mitosis
Development
egg
Duplication of chromosomes
before meiosis starts
M1 M2
M1 M2
Clone of Mother
Development
egg
Failure of
Meiosis II
M1 M2
Offspring inherits half of the mother’s alleles: Half-Clone
Development
egg
M1 M2
Offspring inherits half of the mother’s alleles: Half-Clone
Development
egg
Fusion of Egg
& Second Polar
Body
or chromosome duplication before first mitotic division in development
Homologs
M1 M2
Types of Parthenogenesis – we are considering OBLIGATE parthenogenesis in all-female species
M1 and M2 refer to an entire set of chromosomes; for all female species, there is no paternal contribution
Parthenogenetic eggs can be produced by mitosis or meiosis
Mitosis: offspring are exact clones (except for mutations that occur)
Meiosis: because of crossing over, offspring are not exact copies; but are very similar to the mother; this mode of reproduction tends to preserve heterozygosity
Failure of meiosis II: half clone
Duplication of chromosomes prior to meiosis (results in 2 rounds of replication before a cell division; i.e. the cell that enters meiosis is tetraploid): clone
Fusion of ovum with second polar body – half clone of mother
Examples in snakes, lizards, fish
Clipart from Microsoft Office