1. Sex Determination

2. Sexual Differentiation
The process of development of the differences between males and females, from an un differentiated zygote

3. Sexual Differentiation
Autosomes
Sex chromosomes
Heterogametic sex (2 types of gametes)
Homogametic sex (1 type of gamete)
Males are not always heterogametic sex - females are heterogametic in birds, moths, fish and chickens
XX/XY – male heterogametic sex
ZZ/ZW – female heterogametic sex

4. Sexual Differentiation
Primary Sexual Differentiation – gonads
Secondary Sexual Differentiation – overall appearance of the organism
Unisexual, Dioecious, Gonochoric – containing only male or female reproductive organs
Bisexual, Monoecious, Hermaphorditic – both male and female reproductive organs

5. Sexual Differentiation
Chlamydomonas (Green algae)
Most of life in haploid stage
Asexually reproduce daughter cells by mitotic division
Unfavorable conditions – (such as nitrogen depletion) function as gametes to produce zygotes
Diploid Zygotes – withstand unfavorable conditions

8. Sexual Differentiation
Meiosis occurs then returns to the haploid state
Isogametes – gametes are indistinguishable (isogamous is the species producing the isogametes)
When forming zygotes, only “+” and “-” isogametes mate
Chemical difference between cells but no morphological difference

9. Sexual Differentiation
Zea mays
A. Stamen - Male
1. Produce diploid microspore mother
cells
2. Each mother cell undergoes meiosis
to produce 4 haploid microspores
3. Each haploid microspore develops into
a mature male microgametophyte
(pollen grain)

10. Sexual Differentiation
B. Pistil (Female)
1. Produce diploid megaspore
After meiosis, only 1 haploid megaspore survives
Megaspore divides 3 times producing a total of 8 nuclei in one embryo sac
Two nuclei in the middle – endosperm nuclei

11. Sexual Differentiation
Micropyle end (where two sperm nuclei enter) – 3 nuclei/1 oocyte, 2 synergids
Antipodal nuclei – the three nuclei opposite from micropyle end
One sperm fertilizes endosperm nucleus; the other sperm fertilizes oocyte nucleus
Double fertilization – diploid zygote/triploid endonucleus
Each ear of corn contains as many as 1000 of these structures

13. Sexual Differentiation
C. elegans
Only 1000 cells
2 sexual phenotypes
Males – testis (functional)
Hermaphrodite – XX
Males – X (No Y chromosome)

14. Sexual Differentiation
6. Hermaphrodite – testis and ovaries
a. During larval stage, testis produce sperm (stored)
b. Ovaries produced but no oogenesis until adult stage
c. Able to self fertilize
d. If hermaphrodite mates with male – ½ hermaphrodite, ½ males

17. Sexual Differentiation
Protenor (insect)
1906 Edmund Wilson found female somatic cells contained 14 chromosomes, including 2 X sex chromosomes
Gametes from female contains 7 chromosomes, including 1 X chromosome

18. Sexual Differentiation
Gametes from the male contains 6 chromosomes without 1 X chromosome
Gametes from the male contains 6 chromosomes with 1 X chromosome
Fertilization by male containing
X sperm – FEMALE
6. Fertilization by male containing
O sperm – MALE
Protenor Mode of Sex Determination –
XX / XO

20. Sexual Differentiation
Lygaeus turicus
Insect has 14 chromosomes
12 autosomes and 2 X chromosomes – Female
12 autosomes and 1 X Y - Male
Females produce only X chromosomes
Males produce X and Y chromosomes
Lygaeus Mode of Sex Determination (XX/XY)