Comparative Vertebrate Reproduction

1
Comparative Vertebrate Reproduction

• Part II. Sex and Sexual Differentiation

2
Introduction and Overview

• Acquiring the Sexual Phenotype
• Genetic Basis of Sex Determination
• Hormonal Influences on Sexual Differentiation
• Environmental Influences
• The Sex Ratio
• Sex Allocation

3
Acquiring the Sexual Phenotype

• Two primary mechanisms
• Primary Sex Determination - Gonads form and
  differentiate into testes or ovaries
• Secondary Sex Determination Involves the
• development of extragonal characteristics
• - Primary Sex Characteristics
• - Secondary Sex Characteristics

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Primary Sex Determination Formation of
Indifferent Gonad
Origin of Primordial Germ Cells - hindgut
endoderm as in mammals - various extraembryonic
tissues as in lizards, snakes, and birds
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Gonad Differentiation Organization of the
Mammalian Testis
6
Gonad Differentiation Organization of the
Mammalian Ovary
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Comparative Aspects
Fish Ovary - guppy
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Comparative Aspects
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Secondary Sex Determination Primary Sex
Characteristics
Differentiation of duct systems
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Duct system in male snakes
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Secondary Sex Determination Primary Sex
Characteristics
Control of urogenital system development in
mammals
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Secondary Sex Determination Primary Sex
Characteristics
Differentiation of the genital tuberacle in
humans
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Secondary Sex Determination Secondary Sex
Characteristics

• Modification in body form (changes in allometry)
• Sex-specific modifications in body size and
  proportions
• Changes in coloration
• Changes in behavior patterns

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Secondary Sex Determination Secondary Sex
Characteristics
Examples of vertebrate sexual dimorphism
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Genetic Basis of Sex Determination

• One or Two Specialized Sets of Heteromorphic
  Chromosomes (i.e., homologous heterosomes sex
  chromosomes)
• A heterogametic individual has a 50 chance of
  producing a haploid gamete containing one of the
  two types of sex chromosomes
• - In mammals heterogametic condition (XY)
  results in a male XX or XO females that are
  homogametic
• - In most reptiles and birds, females are
  heterogametic (ZW notation) ZZ/ZO males that
  are homogametic
• (The O means one of the heterosomes is
  absent.)

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Anomalous Chromosomal Combinations In Humans

• XX males have fragments of Y chromosome
  translocated to either X or to the autosomes
• Nondisjunction of sex chromosomes can lead to
  aneuploids lacking a normal chromosomal
  complement (e.g., Klinefelter syndrome XXY)
• Aberrant heterosomal combinations (e.g., Turner
  syndrome XO super female XXX)
• Expression of a gene on the Y sex chromosome is
  responsible for genetic sex determination in
  mammals. The gene (SRY) results in the production
  of a testis-determining factor (TDF)

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Genetic Basis for Sex Determination in Vertebrate
Lineages

• Cyclostomes and elasmobranchs unclear
• Teleosts either XX/XY or ZZ/ZW system
• Amphibians XY (heterogametic)
• Most reptiles ZZ/ZW system
• Birds ZZ/ZO (ZO genotype being normal for
  females)

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Vertebrate sex determination systems. Phylogeny
of major vertebrate clades showing the sex
determining systems found in members of the
respective clade. Multiple indicates
involvement of more than one pair of chromosomes
in sex determination. TSD temperature-dependent
sex determination. (From Ezaz 2006)
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Hormonal Influences on Sexual Differentiation

• Artificial or environmentally-induced alteration
  of one or more of the hormonal signals can alter
  the sexual phenotype expected from a genotype.
• Many non-mammalian vertebrates have an enormous
  susceptibilty to exposure to endocrine disruptors
  in the environment (e.g., American alligators).
  These chemicals (e.g., xenoestrogens or estrogen
  angonists mimics xenobiotics) can have
  epigenetic influences during embryonic
  development or during adult life.

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Environmental Influences

• By eliciting subtle changes in the production,
  reception, and timing of the hormonal signals
  involved in sexual differentiation, environmental
  influences can act as epigenetic factors
  affecting the acquisition of sexual phenotype

Some scientists have attributed recent sex
abnormalities in animals to pollution by these
xenoestrogens. In Britain, for instance, male
trout produced female proteins Great Lakes
salmon grew enlarged thyroids, and in polluted
spots on both U.S. coasts, male gulls were
feminized and females developed an extra oviduct.
This all reflects the basis for the emerging
science of endocrine disruption... "Scientists
have theorized that these abnormalities all
resulted from exposures to chemicals that mimic
or counteract hormones in the body" (New theory
suggests sex in danger. The Osteopath. Summer
1995 1(3)24).
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Social Factors

• Social environment can trigger change in sexual
  phenotype (i.e., sex reversals)

Banded Wrasse, Halichoeres notospilus Female. 
Caught in the surf in December 2007, early
morning at Cabo Real, Km. 21, San Jose del Cabo,
Baja California Sur, Mexico, on cut squid bait,
utilizing a Carolina rig and size 4 Mustad 92553
hook. This species undergoes a sex reversal at
midlife with the females becoming
males. Description and photo courtesy of John
Snow.
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Temperature-Dependent Sex Determination (cont.)
Temperature-dependent sex determination.
Aromatase activity levels during the
thermosensitive period (TSP) are regulated by
the temperature of the environment and control
gonadal differentiation. Changes in the
environment temperature before and after TSP do
not seem to affect sex. Manolakou et al.
Reproductive Biology and Endocrinology 2006
459 doi10.1186/1477-7827-4-59
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The Sex Ratio

• Expressed as the relative proportion of male and
  female phenotypes within a population
• Primary sex ratio Male/female ratio produced at
  the time of fertilization.
• Secondary sex ratio Ratio following completion
  of parental investment in the offspring. (can
  reflect the TSD during development, differential
  pre-partum mortality, and differential mortality
  during post-hatching or post-partum parental
  care.
• Tertiary sex ratio (operational sex ratio) Sex
  ratio representing the relative number of adult
  males and females within a population.
• Ratio commonly expressed as the number of males
  divided by the number of females in a given
  population

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Sex Allocation

• Partitioning of male and female reproductive
  function among different individuals. Different
  patterns of sex allocation reflect the specific
  environmental pressures on a particular lineage
• Gonochorism bisexual phenotypes established
  early in life and persist throughout adult life.
• Hermaphroditism individuals that are capable of
  functioning as both a male and a female during
  the course of their life are said to be
  hermaphroditic or cosexual.

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Types of Hermaphroditism

• Sequential or serial hermaphroditism
  individuals are capable of alternating between
  the production of male and female gametes over
  the course of their lives.
• Sequential hermaphrodites are commonly undergo
  sex reversal. If sex reversal occurs more than
  once during a lifetime, this double sex reversal
  is known as diandry or biandry.
• Simultaneous or synchronous hermaphroditism
  produce functional male and female gametes at the
  same time.

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The Black Hamlet(Hypoplectrus nigricans)

• Having the ability of coordinated alteration of
  sexual phenotype with each member of a mated pair
  switching between the process of the production
  of ova and sperm is termed egg trading.
• - a special type of
• sequential hermaphroditism
• called protogynous herma-
• phroditism protogyny ( initially
• a female) in a species where male
• size impacts fitness.

Black Hamlet
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The Knobbed Porgy(Calamus nodosus)

• Sequential hermaphroditism in which
  differentiation into males occurs first
  protandrous hermaphroditism protoandry
  (adaptive when large males are not successful at
  monopolizing females and when larger females
  produce more ova than do smaller females)
• Occurs among eight
• families of teleosts

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• Normally, synchronous hermaphrodites are not
  self-fertilizing. The only known
  self-fertilizing (selfing) species is the
  mangrove killifish (Rivulus marmoratus).
• Most synchronous
• hermaphrodites possess
• an ovotestis containing
• oogenic as well as
• spermatogenic tissues.

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Ovotestis Organization in Sea Bass
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Induced Hermaphroditism (using drugs)
Right lobed gonad of a treated male R. pipiens
(0.1 ppb atrazine) with anterior testes and
developing posterior ovary. (A) Gonad fixed in
Bouins solution white arrows show areas where
transverse cross-sections were taken. (B)
Anterior portion is testicular, with lobules that
contain spermatids white arrows indicate lobules
with spermatids. (C) Posterior portion of the
gonad has large testicular oocytes. Bar 250 µm
for (AC). From Hayes et al. (2003) VOLUME 111
NUMBER 4 April 2003 Environmental Health
Perspectives
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Ovotestis in TrueHermaphrodites in Humans

• Case 3. Ovotestis in true hermaphrodite.
• A, testicular compartment shows solid tubules
• filled with immature Sertoli cells and germ
  cells
• (arrow). B, ovarian compartment has numerous
• primordial and growing follicles containing
• primary oocytes within the ovarian cortical
• stroma. C, primordial follicles are invested by
• a single layer of flattened follicular cells
  (arrow),
• and each contains a primary oocyte

True Hermaphroditism and Mixed Gonadal
Dysgenesis in Young Children A
Clinicopathologic Study of 10 Cases.
Kyu-Rae Kim, Youngmee Kwon, Jae Young Joung, Kun
Suk Kim, Alberto G Ayala and Jae Y Ro. Mod
Pathol 200215(10)10131019.
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Ovotestis in Humans (continued)
33

• A form of synchronous hermaphroditism, called
  acquired hermaphroditism, occurs in the deep-sea
  ceratioid anglerfish (Edriolychnus schmidti)

The male deep-sea anglerfish attaches himself to
the female body and fuses to it. The male
shares the females blood supply and all systems,
except his reproductive system, atrophy. --- a
unique individualis formed out of
intraspecific parasitism
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• Ambiguous sexual development can produce
  pseudohermaphrodites

Pseudohemaphrodite anatomy of Diamondback
watersnake, Nerodia rhombifer (J. Stanley and S.
Trauth, unpubl.)