This document discusses developmental homeostasis and homozygosity. It defines homeostasis as an organism's ability to regulate itself in response to environmental changes. There are two types of homeostasis - genetic and developmental. Developmental homeostasis refers to an individual's ability to produce consistent phenotypes despite environmental variability. Heterozygous individuals exhibit greater developmental homeostasis than homozygous individuals. Several genetic models are discussed, including the role of additive genetic variation, dominance effects, and epistasis in developmental homeostasis. Maintaining coadapted gene complexes and heterozygosity are important for developmental homeostasis.

1. TERM PAPER PRESENTATION
ON
DEVELOPMENTAL HOMEOSTASIS AND
HOMOZYGOSITY
GOWTHAMI R

2. HOMEOSTASIS
Homeostasis refers to the property of the
organism to adjust itself to variable conditions
or self regulation (genetic, physiological or
morphological) of populations of plants and
animals against any fluctuating and sudden
change in the external environment.
The concept of self regulation was first
suggested by CLAUDE BERNARD Inin 1865.
The term homeosstasis was given by CANNON in
1932

3. TYPES OF HOMEOSTASIS
LERNER in 1954 distinguished two types of homeostasis
1. GENETIC HOMEOSTASIS
2. DEVELOPMENTAL HOMEOSTASIS

4. GENETIC HOMEOSTASIS- is the property of a population of
genotypes to resist sudden changes.
DEVELOPMENTAL HOMEOSTASIS- The ability of
Individual genotypes to adopt themselves to variable
environments and to produce predictable phenotypes
The above terms were also referred as populational
homeostasis and individual homeostasis by LEWONTIN in
1957
Populational buffering and individual buffering by ALLARD
AND BRADSHAW in 1964

5. Heterozygous or heterogeneous individuals are more
homeostatic than homozygous or homogeneous
individuals.
Homeostasis was inversly proportional to the variability
expressed by the individuals

6. OUTCOMES OF DEVELOPMENTAL HOMEOSTASIS
1. CANALIZATION-
is the ability of a genotype to express the same
phenotype across environments.
1. DEVELOPMENTAL STABILITY-
The repeatability of the same character
within a specific environment.

7. QUANTIFYING DEVELOPMENTAL
HOMEOSTASIS
Fluctuating assymetry
Extent to which a trait departs from the
normal individuals

8. VARIATION OF EXPRESSION OF DEVELOPMENTAL
HOMEOSTASIS
1. EMIPRICAL EVIDENCE :- If the evolution of developmental
homeostasis for traits follows a NEO-DARWINISM process
there must be a variation in expression among individuals
2. GENETIC STRESS :- Mutations reduces developmental
homeostasis.
Inbred lines decreases developmental homeostasis.
Hybridisation among related species also results to
reduce developmental homeostasis.
3. ENVIRONMENTAL STRESS :- Environmental factors,
Anthropogenic sources of pollution

9. GENETIC BASIS OF DEVELOPMENTAL
HOMEOSTASIS
The contribution of genetic variation to a trait can be
partitioned into further components reflecting the
different modes of gene action i.e.
a. ADDITIVE COMPONENT
b. DOMINANCE COMPONENT
c. INTERACTION OR EPISTATIC COMPONENT

10. ADDITIVE GENETIC VARIATION
The resemblance between parents & offspring is due to
additive genetic variation
Selection acting on differences among individuals results in
evolutionary response depending on the degree to which
additive genetic variation underlies phenotypic variation.

11. DOMINANCE OR HETEROZYGOSITY
The role of heterozygosity in developmental homeostasis
was first emphasized by LERNER (1954) and it was termed
as PHENODEVIANTS to describe individuals whose
development is more prone to environment fluctuations.
LERNER used an overdominance hypothesis model to
describe the relationship between heterozygosity and
developmental homeostasis.

12. EPISTASIS OR COADAPTED GENE COMPLEXES
Dobzhansky (1970) first suggested that coadapted genes
complexes are responsible for the expression of
developmental homeostasis.
In natural or artificial hybrid populations, coadapted gene
complexes may be by the mixing of the parental genomes.
Therefore, hybrid studies have been used to demonstrate
the presence of coadapted genes.

13. GENETIC MODELS OF DEVELOPMENTAL
HOMEOSTASIS
1. CROW-MULLER MODEL
2. DOZHANSKY MODEL
3. LERNERS MODEL

14. CROW-MULLER MODEL
Heterosis historically pertains to the vigor seen in hybrids,
it is sometimes used interchangeably in the literature
with developmental homeostasis in the sense that
hybrids who manifest true heterosis will also manifest
developmental homeostasis.
High Darwinian fitness is associated with developmental
homeostasis.
Most individuals in a Mendelian population should, then,
be homozygous for most genes. Heterozygous loci will be
a minority.”

15. Heterozygosity in a population occurs because of
(1) Recurrent mutation at each locus coupled with the
interaction of selection,
(2) The presence of genetic variants which are adaptively
neutral, or which possess slight adaptive advantages
at some times in some places,
(3) Adaptive polymorphism maintained by the diversity
of habitats occupied by the population, and
4) the rare good alleles which have not had time to
displace their alleles.

16. DOBZHANSKY MODEL- 1950
Importance of coadapted in the evolutionary process and
for heterosis (developmental homeostasis).
Dobzhansky’s often cited 1950 paper emphasizing the
importance of coadapted heterozygosity in the evolutionary
process and his prior publications promoting neoDarwinism,
The term genomic coadaptation can be defined, therefore,
as the coadapted interactions of the genes in the genome or
genomes present in an organism.

17. The term coadaptation used by Dobzhansky, Lerner, and
certain other biologists, refers to the mutual adjustments
that occur in a gene pool by the action of natural
selection to produce desirable outcomes.
According to Lerner (1959) the process of coadaptation is
probably a continuous one. A change in allelic
frequencies at one locus by selection or by some other
force results in a change of allelic frequencies at
other loci.

18. He proposed there is selection for mutations at loci
within the region of each inversion that result in
overdominance (i.e., the heterozygote may be more fit
then either homozygote because of the greater
biochemical versatility of having the products of two
alleles rather than the products of the same allele).

19. Mendelian populations, by the action of natural selection,
acquire a high level of obligate heterozygosity at loci in
polygenic systems, which become important genetic
mechanisms for developmental homeostasis.
Individuals who are highly heterozygous at the loci
comprising these multigenic (polygenic) systems are
buffered during development, and individuals who are
highly homozygous at these loci may manifest phenotypic
expressions consistent with lack of canalization.
LERNERS MODEL

20. IMPORTANCE OF DH
 DH provide a way to understand how a complex traits are
evolved.
The breakdown of DH is necessary for population to release
variability and to respond to new selection pressure.
Now a days more applicable in plants also because
1. It is easy to quantify the relationship between DH and
fitness
2. The modular construction in plants indicates that the DH
may differ among the levels of organisation within plant
3. Great range of mating and genetic systems found among
plant species helps in quantfying DH

21. CASE STUDY- ESTIMATES OF GENETIC HOMEOSTASIS IN MAIZE
Objective- to estimate genetic homeostasis in maize by using
environmental variance
Materials- P1,P2,F2,F3,BC1,BC2,single crosses
Results- the homogenous inbred lines and single cross entries had
more GXE interaction mean squares than heterogeneous F2, F3, BC1,
BC2
Inbred lines were less homeostatic than the single crosses and F2,
and backcross generations were most homeostatic populations

22. HETEROZYGOTE SUPERIORITY AS DISCUSSED BY LERNER
1954 IS THE MAJOR EXPLANATION FOR BETTER
HOMEOSTASIS IN POPULATIONS CONTAINING HIGHER
LEVELS OF HETEROZYGOSITY OVER THOSSE WITH NONE
OR REDUCED LEVELS OF HETEROZYGOSITY