3. • These repetitive sequences often colonize specific
chromosomal or sub chromosomal niches.
• In non recombining regions of the Y chromosome
repetitive DNA sequences are accumulated (Role
in genome ecosystem and shape of Y-
chromosome)
• Which represents a dominant and early process
forming the Y chromosome, before genes start to
degenerate, which leads to formation of
heteromorphic sex chromosomes
• Ex- 1st report on Silene latifolia, Carica papaya.
4. Sex chromosomes: special parts of
genomes
• The conversion from autosomes to sex
chromosomes may start with the emergence of a
sex-determining gene with one allele that
determines male individuals and the other
female individuals.
• According to two loci model by Charlesworth
et al. (2005)
•Proto X mutation – Male sterility
•Proto Y mutation – female sterility
5. • Later, other sex-determining genes that
influence the development of a particular sex,
or are antagonistic to the opposing sex, may
accumulate around the sex determining gene.
• Such sex-determining regions can, in some
instances translocate between chromosomes
and create new sex chromosomes with non
recombining regions.
• Factor 1. Sex determining locus accumulation
2. Sex-determining genes accumulation
3. Antagonistic gene to opposing sex
6. • It is not clear yet whether the processes of
gene degeneration, TEs accumulation and
expansions on one hand and contractions on
the other hand are stepwise or are occurring
simultaneously.
• Non-recombining regions may expand through
the accumulation of repetitive DNA sequences
,which often form heterochromatin
7. Process acting in regions of reduced
recombination
• There are 3 models describing the population
genetics of TEs in regions of reduced recombination
1. Deleterious insertion model is based on the higher
elimination of TEs from high-gene density regions
2. Ectopic recombination model explains the higher
abundance of TEs in low recombining regions by the
reduced frequency of their removal by ectopic
recombination
3. Deleterious transposition model is based on the
deleterious effects of transposition, for example, the
formation of chromosome breaks.
8. Contd……..
• Centromeres are regions with lower
recombination frequency and exhibit satellite
sequences and TEs, as in cereals.(Arabidopdis)
• Centromeric satellite repeats may arise from DNA
transposons by evolution.
• Centromeric satellites STAR-C and STAR-Y are
represented in centromeres of the Y
chromosome in Silene latifolia.
• Gene conversion and gene degeneration also has
role in non recombining regions ,which makes
epigentic regulation of genes ex- Papaya MSY
gene.
9. Accumulation of repetitive DNA in plant sex
chromosomes
Est.of sex
determining region Degeneration
of other gene
Shrinkage by
deletion
Accumulaton of
repeats DNA
Homomorphic sex chromosome Heteromorphic sex chromosome
Ex-1. Squirting cucumber (Ecballium elaterium)
2. Asparagus officinalis
3. kiwi fruit (Actinidia chinensis)
4. Carica papaya-MSY
Sex chromosome evolution
10. Contd……..
• Heteromorphic sex chromosome: (dioecious)
• Ex-1. liverwort( Marchantia polymorpha)- Y chromosome of liverwort has specific
repeat sequences that contain multiplicated genes in male.
• 2. white campion (Silene latifolia) - Y chromosome is the largest chromosome
in the karyotype and is mostly nonrecombining with the X chromosome
• 3. sorrel (Rumex acetosa) - Sorrel Y chromosomes contain two types of satellite
DNA called RAE180 and RAYS
• 4. hop (Humulus lupulus) - the Y chromosome is the smallest chromosome in
the genome.
• 5. hemp (Cannabis sativa) – Hemp shows male-specific LINE-like
retrotransposons as well as a high abundance of MADC3 and MADC4 (male-
associated DNA sequences), encoding gag/pol polyproteins of copia-like
retrotransposons .
• 6. ivy gourd(Coccinia grandis)
11. Silene latifolia : a model dioecious plant
with heteromorphic sex chromosomes
• Silene latifolia (2n=24 white campion)
• Not only in size but also in DNA composition X
and Y chromosomes are vary.
12.
13. Conclusions and prospects
• Sex chromosomes are more diagrammatic than dynamic as
we seen.
• The contribution of many processes such as accumulation
of repetitive DNA, degeneration of genes, additions of
genes, small or large deletions, inversions are not clear
neither the timing.
• We are also far from understanding the dominant
processes working on the Y chromosome such as the
preferential accumulation of repetitive DNA in regions with
suppressed recombination.
• How, non-recombining regions of plant sex chromosomes
degenerate not clear. So we need to have a lot of machinery
before conclude the evolution and the role of repetitive
DNA in it
