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Haplodiploidy
Males are haploid (1N) and develop by
parthenogenesis.
Females are diploid (2N).
Ex. Honey bee (order Hymenoptera)
Workers and queens are 2N, whereas drones are
1N.
Parthenogenesis: When unfertilized eggs may
become activated and develop into viable individuals
as part of the normal life cycle.
• Parthenogenesis may occur in a number of invertebrate
groups (i.e. insects) as well as in scattered vertebrate
species (some fishes, a few lizards, and even turkeys).
• Ex. Beltsville small white turkeys
Parthenogenesis: Development of an ovum into a
new individual without fertilization.
• In turkey eggs, parthenogenetic development can be initiated
spontaneously. Early embryonic development in parthenogens is often
delayed. Only 20 % of the parthenogens that complete embryonic
development survive to produce offspring. In parthenogenetically
developing blastoderms, approx. 40.3 % haploid and 48.9 % diploid
cells are found. The haploid cell complement of parthenogens declines
to 1.5 % at 29 d of incubation, 1.4 % at 4 wk posthatch, and 1.3 % in
adulthood. The semen volume of male parthenogens is reduced.
However, their germ cells are capable of normal meiosis and sperm
maturation leading to normal fertility. The mechanism through which
diploidy is obtained has not yet been elucidated, however, the diploid
number could be the result of the union of the haploid egg nucleus and
the second polar body or the lack of meiosis II.
Sex of Parthenogens in Birds and Mammals
Mammal
Bird
XY
1st polar
body Y
XX
XX
X
2nd polar
ovum
X
X
body
recombine
XX
Combine
(artificially induced)
X
2º oocyte
X
2º oocyte
XX
(embryo dead by day 10 of gestation)
Protenor sp.
X and Y chromosomes are found in most insects, all
mammals (i.e. humans), in many fish and amphibia,
and in a few plants.
Male
Female
Chromosome constitutions (karyotypes) of males and females
in Drosophila melanogaster.
Calvin Bridges model for sex
determination
in Drosophila melanogaster
• “A” = a set of 3 autosomes
• Therefore, a normal fertile female having 8
total chromosomes is designated as 2A2X,
and a normal fertile male having 8 total
chromosomes is designated as 2AXY.
P
AAXX
A AXX
x
AAXY
AX AY
Result of nondisjunction
in Anaphase I
(AAXXY)
2:2
XA
Normal Female
(AAXXX)
3:2
XA
Meta Female
(AAXO)
1:2
XA
Sterile Male
(AAOY)
0:2
XA
Dead
Calvin Bridges model for sex determination
in Drosophila melanogaster
X > 1 = metafemale
A
X = 1 = fertile female
A
0.5 < X < 1 = intersex
A
X = 0.5 = male; + Y = fertile male
A
0 < X < 0.5 = metamale
A