7.1 General sex determination

In metazoans (animals), how are eggs and sperm defined, regardless of other morphological differences?

• Egg: the larger, immobile gamete

• sperm: the smaller, mobile gamete

Metazoan sperm and egg

• evolved only once, prior to radiation

ex: females have larger gametes (eggs) and males have smaller gametes (sperm)

Anisogamous

• an organism whose gametes have different morphology

Isogamous

• gametes that look identical from the outside, but can be different genetically (mating type)

ex: yeast spores

Sex determination

• for anisogamous organisms

• 3 possible sexes

• expressed in the diploid stage or the haploid stage

• can vary in the length of their non-recombining regions

• involved in pathways that are important to developing the germline of the organisms

Homothallism

the capability to mate with your own mating type

heterothallism

• the capability of mating with a different mating type

• can have more than 3 possible mating types

hermaphrodites (Hermaphroditism)

• They have both sexes, or the sexes can differentiate into male and female

• They are capable of self mating

  • during mating, a pair takes turns acting as male and as female for multiple matings

  • in clown fish: the female is largest and the others are males; if the female dies, then the largest male turns into a female

Diploid sex determination

• expressed only in the diploid stage

• sperm can be X or Y (XY or ZW)

• sexual dimorphisms are observed

Haploid sex determination

• the difference is expressed/observed only in the gametes

• can be UV

Homogametic sex

• the biological sex that has two matching or similar sex chromosomes and produces only one type of gamete regarding those chromosomes

• Female XX (Mammals)

• Male ZZ (Birds)

Heterogametic sex

• the biological sex that produces two different types of gametes (sperm or egg cells) with respect to sex chromosomes

• Male XY (Mammals)

• Female ZW (Birds)

Y chromosome recombination:

• The X chromosome is capable of recombination in females

• used to never experience recombination

pseudoautosomal regions (PARs

at the terminal ends of the mammalian Y chromosome that recombine with the X

Heteromorphic sex chromosomes

• will tend to not recombine across most of their length, with small pseudo-autosomal regions (PARs) occasionally being the exception

Homomorphic sex chromosomes

• aka sex loci

• have small regions of no recombination, where the rest of the chromosome behaves as an autosome

• can evolve from autosomes

How do heteromorphic sex chromosomes evolve?

• A new masculinizing mutation arises on the chromosome that will become the new Y

• A second masculinizing mutation arises on the same chromosome

• Selection leads to the loss of recombination between the two loci

• Both masculinizing mutations are then always inherited together

Why does the Y chromosome shrink over time

• New masculinizing mutations arise and spread the non-recombining region

• The Y chromosome cannot recombine in these regions

• It is less effective at fixing structural mutations

maintaining a PAR allow…

the Y chromosome to be stable

Sex loci

• can arise from novel sexualizing mutations

• can then turn into a sex chromosome over time due to selection to keep sexualizing mutations on a single chromosome

Recombination stops in the middle of the chromosome here because having these two alleles inherited together conveys an advantage. A mutation to the underlying satellite that guides recombination is the typical cause. The population not having recombination here is because
the underlying mutation increases in frequency. What type of selection does the mutation that causes this lack of recombination experience?
A) Positive B) Negative
C) Balancing D) Drift

A) Positive

Haplodiploidy

• occurs when one sex is diploid and the other is haploid

• Happens in a few insects

  • where the diploid sex is female and the haploid sex is male

• male sperm is made through mitosis

  • only in species that host Wolbachia

Paternal genome elimination

• paternal genome being removed at some point of development

• can have a similar phenotype to haplodiploidy, but is controlled very differently, and is more widespread

• from full elimination during embryogenesis, to only eliminating the paternal genome in meiosis 1 of males

Environmental sex determination

• when an outside factor like temperature determines the sex of an individual

• presence of a female can determine sex in some species

Autosome Balance Determining Systems in Drosophila

• XX female and XY male (+ 3 pairs of autosomes)

• The Y chromosome does not determine sex.

• The ratio of X to autosomes (X:A) determines the expression of Sxl

  • a gene that signals the female sex determination pathway

Master sex determination gene

• the situation where the presence of a single gene promotes the development of a sex

ex: SRY: its presence will always lead to testes and male development

Parthenogenesis

• females laying eggs that develop into zygotes without paternal contribution

• Can lead to all female species

What do many parthenogenic species still require

• input from a related male species for sperm activation for proper zygote formation

• The paternal genome is normally kicked out of the zygote and not allowed to contribute

• In rare cases the paternal genome is left in and the zygote is triploid

What sex do you expect XXXY humans to be?
A) Male
B) Female
C) Intersex
D) Hermaphrodite

A) Male

What sex do you expect XXYY Drosophila to be?
A) Male
B) Female
C) Intersex
D) Metasex

B) Female

Bluehead Wrasse is a coral reef fish that lives in groups that are led by a large dominant male and a cohort of smaller females. When the male dies, one of the females grows testes and larger in size and becomes a new dominant male. What type of sex determination is this?
A) Simultaneous hermaphroditism
B) Sequential hermaphroditism
C) Environmental sex determination
D) Autosome balance system

B) Sequential hermaphroditism

What kinds of pathways do sex determination genes often fall into,

• fall into similar pathways involved in either development of the germline or dictating the sex determination

• can affect things such as ornamental traits that increase mating for one sex

dm-domain

• containing genes are transcription factors that generally promote male differentiation in Metazoa

• often chosen as sex determination genes (chickens and Xenopus)

• bias because they are already involved in the sexual differentiation pathway, so there are fewer barriers for them to be chosen as the novel sex determination gene

Sox genes

transcription factors involved in development, and are the family that gave rise to SRY

SRY genes

• specifically stops normal SOX3 from inhibiting testes development, thus promoting testes development

ex: dominant negative mutation

XX/XO systems

• there is only one sex chromosome that is either homogametic or hemigametic

• depends on sex chromosomes: autosome ratio (either X or Z for sex chromosome)

ex: C. elegans —> XX hermaphrodites and XO males

C. elegans embryos

• start off as male and swap to female development (under certain conditions)

• opposite of what humans do, where all embryos are female unless sry is present and functional

What happen if you increase the number of X chromosomes and autosomes equally?

the sex stays the same in these systems

gynandromorphs

• whose sex is different in different parts of the body

• can be created through genetic manipulation

Both XY/ZW and XO/ZO systems

• can develop sex determination systems based on rations between sex chromosomes and autosomes

• often still have sex determination loci (tra-1 and sxl), not a singular factor promoting sex determination

Metasex individuals

• those with greater or smaller sex ratios than the standard

• can produce offspring, but those offspring are often sterile

Mammal’s X chromosome

• heavily methylated and treated with other silencing signals

• controlled by non-coding RNAs that ensure the process only occur when there are multiple …

• always silencing n-1 chromosomes where n is the number of …

• when intersex individuals have multiple … they can still inactivate the extra copies

Mammals differ in their mechanism of inactivation

• Some have random inactivation in different cell types and can lead to phenotypes (like calico cats)

• In rodents and primates, inactivation occurs during development in the blastocyst stage

ex: Opossum (model marsupial) specifically inactivate their paternal X chromosome only

ex: Mammals differ in what regions of the Barr body remain actively transcribed