Sex Determination

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

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

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

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

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)

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

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

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

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

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

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

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

Sexual Differentiation Humans Tjio and Levan (1956) – discovered 46 diploid for humans; 22 pairs of autosomal chromosomes and 1 pair of sex chromosomes

Sexual Differentiation in Humans All human embryos undergo a hermaphroditic period 5th week of gestation, gonadal primordia arise Primordail germ cells become cortex/inner medulla Cortex – develop into ovary Medulla – develop into testis

Sexual Differentiation in Humans 7th week of gestation, if XY chromosomes are present, medulla develops into testis. If no Y present, cortex forms ovarian tissue Initiation of testis stimulates production of two hormones that is needed for male sex differentiation Males produce spermatocytes at puberty Females arrest eggs in meiosis by 25th week

Sexual Differentiation in Humans Y chromosome in Male Development Y chromosome was once believed to contribute very little to the makeup of males (Wasteland theory) Pseudoautosomal regions (PARS) – regions on Y chromosome that share homology with X chromosome These regions synapse/located on ends of Y chromosome

Sexual Differentiation in Humans All the rest of Y is called NRY (nonrecombining regions) TDF – testis determining factor SRY – sex determining region Y (encodes for TDF) Females X Y – missing SRY region on the Y chromosome Males X X – attached SRY region to the X chromosome

Sexual Differentiation in Humans Transgenic Mice studies 1. X X fertilized eggs injected with SRY gene produced male mice 2. SRY is the gene to determine maleness 3. Conserved throughout a diverse group of animals

Sexual Differentiation in Humans David Page 1. Discovered 12 genes on the NRY 2. Two groups a. 5 NRY genes, homolog on the X chromosomes (housekeeping genes) b. 7 NRY genes, No homolog on the X chromosome encode proteins only expressed in testicular tissue

Sexual Differentiation in Humans Dosage Compensation Females (XX) should have a “genetic dosage” problem for all X-linked genes Potential for females to produce twice as much X-linked gene products

Sexual Differentiation in Humans Dosage Compensation Barr Bodies – Murray Barr/Ewart Bertram’s experiments with female cats Noticed a darkly stained body near the nuclear envelope of interphase cells Stained positive with Feulgen stain Dark body composed of inactivated X chromosome (suggested by Susumo Ohno)

Sexual Differentiation in Humans 1. Hypothesis for dosage compensation – one X chromosomes functional between males and females 2. No matter how many X chromosomes are present in females, all but 1 is inactivated (multiple Bar Bodies)29

Sexual Differentiation in Humans Problems with theory If one X chromosome is inactivated, why isn’t the Turner, 45X normal? Why aren’t people with multiple X karyotypes normal? Explanation Not inactivated during early development of gonadal tissues or not all of the X is inactivated

Sexual Differentiation in Humans Lyons Hypothesis 1. Answered the question: Which X chromosome is inactivated? Paternal or maternal 2. Mary Lyon/Liane Russell proposed hypothesis of random inactivation

Sexual Differentiation in Humans Experiment with Female Mice -heterozygous for X-linked coat color 1. Pigmentation on X chromosome (expressing in patches) Females have different patterns based on which X is inactivated Males have only one type of patch expressed