Phenotypic expression of sex

1
Phenotypic expression of sex

• Sex determination
• - a genetic/environmental process involving the
  expression of one or more sex-specific regulatory
  genes that determine specific physiological
  states
• Sexual differentiation
• - a physiological process involving the
  co-ordinated phenotypic expression of sex leading
  to the maturation of ovaries or testes

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How is sex determined?

• Kosswig (1935), Winge (1934)
• Polygenic theory of sex determination
• Major autosomal chromosome pair carrying sex
  determining locus
• May have modifying loci
• May have multiple major sex determining loci

3
Sexual Modes

• One dominant locus in gonochorists
• XY system
• Male heterogametic (XY)
• Female homogametic (XX)
• ZW system
• Female heterogametic (ZW)
• Male homogametic (ZZ)

4
Sexual modes

• Major locus but with autosomal modifiers
• - polygenic with dosage dependence
• - several loci on one chromosome or scattered
  across chromosomes
• Gynogenetic all-female
• no incorporation of male genome, sperm used for
  the initiation of cleavage
• Hybridogenetic all-female
• male genome incorporated but removed at meiosis

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Natural gynogenesis
6
Natural gynogenesis
7
Natural hybridogens
8
Sexual modes

• Simultaneous hermaphrodites
• eg. Rivulus marmoratus
• Protandrous hermaphrodites
• (Sequential) - eg. Amphiprion clarkii
• Protogynous hermaphrodites
• (Sequential) - eg. Thalassoma bifasciatum
• -Devlin and Nagahama (2002). Appendix A

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Sexual modes

• Diandric
• Produce primary males bisexually
• Produce secondary males through sequential
  hermaphrodism
• eg. Lates calcarifer
• Produce secondary males through simultaneous
  hermaphrodism
• eg. Rivulus marmoratus

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Ovotestis in Rivulus marmoratus
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Sexual modes

• Reversible sequential hermaphrodites
• Can initiate sexual maturation as either male or
  female and then reverse.
• eg. Several species of Gobiidae
• Japanese hawkfish (Cirrhitichthys aureus)

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Why evolve different sexual modes?

• Size-advantage model
• expected number of offspring differ between the
  sexes dependent upon their size
• 2) individuals that change sex will leave
  behind more offspring than if they remained the
  same sex throughout their lives

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Factors determining reproductive success?

• Male vs female?

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Monogamous species
Female Male
11 sex ratio
Fitness
Size
15
Protandrous
Female
Male
Fitness
Size
16
Protogynous
Male
Female
Fitness
Size
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Sexual modes

• Abnormal hermaphrodites or intersexes
• Occur in a very small percentage of individuals
  from normally gonochoristic species.
• Characterized by functional ovo-testis.
• See section 3.3 in Devlin Nagahama (2002)

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Changes to sex chromosomes

• Additions / deletions of of heterochromatic
  blocks
• Reductions in chromosome size
• Increases in chromosome size
• Chromosome rearrangements (translocations)

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Polymorphisms in sex chromosomes
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Sex determination

• poorly understood
• In mammals Sry gene family main trigger
• One major locus (DMY) identified in medaka
  (Oryzias latipes)
• Shows affinity to DMRT1 family of genes
• Other HMG-like genes may be possible candidates

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Postulated functions

• Suppression of aromatase activity (Y-linked)
• Testosterone / ? Estradiol
• Enhancement of aromatase activity (W-linked)
• Testosterone ? ? Estradiol
• Modulation of receptor sites

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Detection of sex determining loci

• Induced gynogenesis or androgenesis
• Gynogenesis
• sperm irradiated with UV
• Androgenesis
• egg irradiated with UV or more typically higher
  ionizing radiation

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Detection of sex determining loci(One major
locus)

• Induced gynogenesis
• (forced retention of polar body II (meiogens) or
  suppression of 1st mitotic division (mitogens))
• In XY gonochorists all-female progeny produced
• In ZW gonochorists predominantly female sex
  ratio expected in meiogens if sex locus is
  telomerically located. If sex locus is proximal
  to the centromere in meiogens, or in all
  mitogens, an equal sex ratio may result if WW
  homozygotes are not lethal. Otherwise, only ZZ
  males produced.

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Detection of sex determining loci(One major
locus)

• Induced androgenesis
• (suppression of 1st mitotic division (mitogens)
  using heat shock or pressure-similar to
  gynogenesis).
• In ZW gonochorists all-male progeny produced.
• In XY gonochorists An equal sex ratio may
  result if YY homozygotes are not lethal.
  Otherwise, only XX females produced.

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Detection of sex determining loci(Major locus
with autosomal modifiers)

• Induced gynogenesis
• In ZW gonochorists should produce primarily
  female offspring ratio F M ?
• In XY gonochorists should produce primarily
  female offspring ratio F M ?
• (likely gt 90)

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Detection of sex determining loci(Major locus
with autosomal modifiers)

• Induced androgenesis
• In ZW gonochorists should produce primarily
  male offspring ratio F M ?
• (likely gt 90)
• In XY gonochorists should produce primarily
  male offspring ratio F M ?

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Detection of sex determining loci

• Hormonal treatment to induce sex reversal
• eg. Masculinization of females with
  methyl-testosterone treatment.
• See Tables 1 2 Devlin Nagahama (2002)

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Detection of sex determining loci

• Sex linkage studies identifying sex-linked
  genetic markers
• 11 segregation between a genetic marker in
  either the male or female parent with the sex of
  their offspring
• Phenotypic markers
• Protein markers
• DNA markers

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Sex linkage groups in 4 salmonid species
Brown trout
Atlantic salmon
Arctic charr
Rainbow trout
Woram et al. 2003. Genome Res. 13272-280
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Detectection of sex-specific DNA markers using
Fluorescent in situ hybridization (FISH)
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Gonad development

• Germinal ridge produced from mesodermal layer
• Maternal mRNA directs development of primordial
  germ cells (PGCs-endodermal)
• In zebrafish vas gene expression associated with
  4 cells in the 1000 cell blastula
• At the gastrula stage 30 cells in the germinal
  ridge are vas-positive.
• PGCs are totipotent and sexually undifferentiated.

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Gonad development

• Somatic cells (Sertoli, Leydig, thecal, granulosa
  layers) supporting gonads derived from epithelial
  layers.
• Ovarian development initiated first.
• In salmonids no sexual differentiation of gonads
  detectable at hatching. At 1 month post-hatch,
  ovarian follicles detectable.

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Primary gonorchorists

• Differentiated gonorchorists
• - PGCs develop directly as either testis or
  ovaries.
• eg. most gonochoristic species
• Undifferentiated gonorchorists
• - development of ovary followed by
  differentiation into testes in males.
• eg. Barbus tetrazona and Danio rerio

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Secondary gonorchorists

• Intersexual gonorchorists
• - produce a functional gonad containing
  primordial eggs and sperm - suggests the species
  is hermaphroditic.
• - mature as a single sex.
• eg. Anguilla anguilla and A. japonica

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Hermaphrodites

• Reversal of functional gonad into the gonadal
  tissue of the opposite sex during the lifetime of
  an individual.
• A) stimulation of previously sex-determined
  cells?
• B) initiation of new cell lines during reversal?
• C) re-organization of existing cellular
  structures into the germinal line of the new sex?

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Sex reversal in the hermaphrodite wrasse
Thalassoma dupperrrey
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Cellular fates?

• Recruitment of undifferentiated cells supported.
• Betta splendens
• 40 of ovarectimized females could be hormonally
  stimulated to turn into males (Noble Kumpf,
  1937 Kaiser Schmidt, 1951).

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Sexual differentiation

• Fish species generally labile throughout their
  lifetime with respect to sexual differentiation.
• Sex easily reversed with hormonal treatment
  (generally not permanent).
• Environmental factors (eg. temperature, salinity,
  chemicals) can modify sex.

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Major sex steroids

• 17? estradiol (females)
• testosterone, 11-ketotestosterone (males)
• 17?, 20 ?-dihydroxy-4-pregnen-3-one (17?, 20
  ?-DP) (maturation hormone)

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Major sex steroids

• 17? estradiol (females)(granulosa layer)
• 11-ketotestosterone (males)(Leydig cells)

aromatase activity in O. niloticus
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Sex determination switch?

• Unknown in fish.
• Mammalian model indicates activity of Sry gene
  necessary.
• Sry switches default female mode into male
  trajectory.
• female-first model supported in fish by earlier
  differentiation of female gonads.

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Regulation of sexual differentiation

• Quantitative levels of the steroids.
• eg. 100x greater level of 11?-hydroxylase in
  rainbow trout testis compared to ovary.
• Varying distribution and number of estrogen and
  androgen receptors.

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Triggering of sexual maturation

• Decline in conditioning steroid hormones.
• Elevation in 17?, 20 ?-DP, prostaglandins, and
  regulation by G-proteins.
• Cyclin B protein is derepressed and complexes
  with cdc-2 proteins to produce maturation
  promoting factor (MPF) protein complexes.

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Neuroendocrine control

• Gonadotropin I (GtHI) FSH
• Gonadotropin II (GtHII) LH
• GtHI ? 11?hydroxylase ? 11ketotestosterone ?
  activin? ? spermatogonia proliferation
• GtHII ? 11?hydroxylase? aromatase ?

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Neuroendocrine control

• Gonadotropin I (GtHI) FSH
• Gonadotropin II (GtHII) LH
• Females
• GtHII receptor proliferation ? GtHI receptors ?
  estradiol ? aromatase ?
• Enzymes required for 17?, 20?-DP ?
• Males
• GtHII receptors only present at spermiation in
  Leydig cells
• Associated with 17?, 20?-DP ?

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Breton, B., M. Govoroun, T. Mikolajczyk, 1998.
Gen. Comp. Endocrinol. 111 38-50
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Breton, B., M. Govoroun, T. Mikolajczyk, 1998.
Gen. Comp. Endocrinol. 111 38-50
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Breton, B., M. Govoroun, T. Mikolajczyk, 1998.
Gen. Comp. Endocrinol. 111 38-50
50
Breton, B., M. Govoroun, T. Mikolajczyk, 1998.
Gen. Comp. Endocrinol. 111 38-50
51
Breton, B., M. Govoroun, T. Mikolajczyk, 1998.
Gen. Comp. Endocrinol. 111 38-50
52
Sexual maturation in rainbow trout

• GtH II appears to signal final maturation pulse
• GnRH more effective in elevating GtH II levels
  than GtH I levels
• Post-ovulatary signals important.
• GtH I levels ? GtH II ? if eggs not present
  (next cycle begins)
• GtH II remains elevated if eggs present

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Control of Reproduction
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Is melatonin important?

• Melatonin elevated during dark cycle
• Varying circannual levels may set clock
• Can reset clock by varying photoperiod
• In salmonids summer day/night cycle in winter
  induces earlier spawning

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• Randall, C.F., and N.R. Bromage,
• Photoperiodic history
• determines the reproductive response
• of rainbow trout to changes in
• daylength. J. Comp. Physiol. A
• 183 651-660.

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Is melatonin important?

• Amano et al. 2000. Gen. Comp. Endocrinol. 120
  190-197
• Amago salmon aritificially fed melatonin during
  high lightdark cycle (normally suppresses mat.)
• Found slight elevation in GSI (gonadosomatic
  index) in test group vs. controls.
• Test group and controls did not differ in spawn
  timing.
• GtH I levles in melatonin group but no
  differences observed in GtH II levels.

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Is melatonin important?

• Melatonin may partially regulate spawning cycle,
  but it does not appear to be the major trigger.