M10.2_Variations_In_Chromosome_Number_W2024_FINAL (2).pdf

BIOL 2030 Module 10 10.2 Variations in Chromosomal Number: aneuploidy & polyploidy Octoploid… Wednesday, March 20, 2024 Heptaploid, 437 chromosomes! Hexaploid… M10.2 1 Sabrina had an unusual number of chromosomes. Why was her condition so extremely rare? M10.2 Why is polyploidy in agricultural crops...

BIOL 2030 Module 10 10.2 Variations in Chromosomal Number: aneuploidy & polyploidy Octoploid… Wednesday, March 20, 2024 Heptaploid, 437 chromosomes! Hexaploid… M10.2 1 Sabrina had an unusual number of chromosomes. Why was her condition so extremely rare? M10.2 Why is polyploidy in agricultural crops both a good thing and a bad thing? Image credit: Wikipedia M10.2 Variations in Chromosome Number Aneuploidy - increase or decrease in the number of individual chromosomes, e.g. trisomy, three copies of a chromosome. Polyploidy – increase in the number of sets of chromosomes, e.g. triploid, three copies of every chromosome. Note difference in terms: trisomy vs. triploidy ‘ploidy’ refers to the total number of chromosomes ‘somy’ refers to the number of particular chromosomes M10.2 2 Aneuploidy: examples compared to normal human genome Normal human diploid individual, 2n = 46 47 (2n+1) - Gain of a single chromosome 48 (2n+2) - Gain of two homologous chromosomes 44 (2n-2) – Loss of both members of a pair of homologous chromosomes 45 (2n-1) - Loss of a single chromosome M10.2 3 Aneuploidy: terminology The four most common types of aneuploidy in diploid (2n) individuals… Nullisomy - Loss of both members of a pair of homologous chromosomes: 2n-2 = 44. Monosomy - Loss of a single chromosome: 2n-1 = 45. Trisomy - Gain of a single chromosome: 2n+1 = 47. Tetrasomy - Gain of two homologous chromosomes: 2n+2 = 48. M10.2 3 Aneuploidy variations… Altered chromosomes 44 (2n – 2) non-homologous Double monosomic Double trisomic (less common) (less common) homologous Nullisomy M10.2 48 (2n + 2) Tetrasomy 4 Origins of Aneuploidy 1) Nondisjunction in meiosis or mitosis. Nondisjunction – failure of homologous chromosomes or sister chromatids to separate 2) Deletion of a centromere leads to chromosome loss. M10.2 5 Origins of Aneuploidy: Nondisjunction M10.2 6 Origins of Aneuploidy: nondisjunction Gametes: Fertilization with haploid gamete (n) Zygotes: 2n+1 Trisomic 2n-1 2n+1 2n-1 2n Monosomic Trisomic Monosomic normal OUTCOMES: Trisomy: may be viable Monosomy: usually not viable, except for sex chromosomes M10.2 7 Aneuploidy: humans Autosomal aneuploidies trisomy 13 Patau syndrome; about 1 in 16000 newborns trisomy 18 Edwards syndrome; about 1 in 5000 liveborn infants trisomy 21 Down syndrome; 1 in 800 newborns Sex chromosome aneuploidies monosomy X (XO) Turner syndrome; 1 in 2500 newborn girls Extra copies of the X chromosome (e.g. XXY-most common, XXXY) Klinefelter syndrome; 1 in 500-1000 newborn males Chromosomal abnormalities, particularly autosomal trisomy, is thought to be the most common cause of spontaneous abortions or miscarriages. M10.2 Source: https://ghr.nlm.nih.gov/; Cytogenet Genome Res. 2017;152(2):81-89 8 Primary Down Syndrome Trisomy 21: 3 copies of chromosome 21 (2n+1 = 47 chromosomes) Accounts for most cases of Down syndrome. Most cases arise from random nondisjunction during meiotic division. Mother contributes the extra chromosome in ~75% of cases. M10.2 9 Primary Down Syndrome Most trisomies are maternal in origin. The incidence of trisomy 21 rises sharply with increasing maternal age Why? Possibly due to the fact that oocytes (eggs) are formed by birth, in arrested stage of meiosis. M10.2 10 Familial Down Syndrome Karyotype of individual with Down Syndrome 21 15 21 An extra copy of chromosome 21 is attached to another chromosome (e.g. 14 or 15). Account for 3-4% of cases. Arise in offspring of parent who carry a chromosome that underwent Robertsonian translocation (= exchange of long arms of non-homologous acrocentric chromosomes) M10.2 11 Familial Down Syndrome Translocation carrier 45 chromosomes, one of which is a translocation chromosome. Normal phenotype, does not have Down syndrome. Karyotype of parent - translocation carrier 21 15 21 M10.2 12 Familial Down Syndrome Normal Father Translocation carrier What would this carrier’s gametes look like? 21 21 15 Normal Mother Meiosis 15 21 21 15 2x 21 15 + 21 15 2x M10.2 12 Aneuploidy: humans – notice how frequency varies with chromosome number Autosomal aneuploidies trisomy 13 Patau syndrome; about 1 in 16000 newborns trisomy 18 Edwards syndrome; about 1 in 5000 liveborn infants trisomy 21 Down syndrome; 1 in 800 newborns Sex chromosome aneuploidies monosomy X (XO) Turner syndrome; 1 in 2500 newborn girls Extra copies of the X chromosome (e.g. XXY-most common, XXXY) Klinefelter syndrome; 1 in 500-1000 newborn males Chromosomal abnormalities, particularly autosomal trisomy, is thought to be the most common cause of spontaneous abortions or miscarriages. Live birth usually have trisomy of smaller chromosomes (e.g. 21) or sex chromosomes. M10.2 Source: https://ghr.nlm.nih.gov/; Cytogenet Genome Res. 2017;152(2):81-89 8 Trisomy 21: Down Syndrome Chromosome 21 is one of the smallest chromosomes M10.2 Sabrina had trisomy of chromosome 9 She was 21 years old in this picture The likely reason that she survived for many years is that the trisomy was present as a mosaic (not all cells had it) M10.2 Aneuploidy: plants tolerate it better than animals Usually viable; phenotype maybe altered and fertility reduced. Jimsom weed (aka loco weed/devil's snare/mad hatter…) a hallucinogenic plant. 2n +1 = 25 Trisomy of individual chromosomes M10.2 Aneuploidy - increase or decrease in the number of individual chromosomes, e.g. trisomy, three copies of a chromosome. Polyploidy – increase in the number of sets of chromosomes, e.g. triploid, three copies of every chromosome. M10.2 14 Polyploidy For diploid (2n) individuals, polyploidy is the presence of more than two sets of chromosomes. Triploids - 3n; Tetraploids - 4n; Pentaploids - 5n; and so on…… Common in plants, less common in animals (some fishes, reptiles, amphibians and invertebrates). Not known in mammals and birds; presumably lethal. Polyploidy is very important in plants. 30-35% of Angiosperms evolved via some form of polyploidy. M10.2 15 Two Types of Polyploidy 1) Autopolyploid – Multiples of the same genome. e.g., autotetraploid - 4n 2) Allopolyploid – Multiples of closely related genomes e.g., allotetraploid - 4n; 2n from species i and 2n from species ii M10.2 16 Origins of Autopolyploidy can occur during mitosis or meiosis Nondisjunction of ALL chromosomes during mitosis in early embryo can produce autotetraploid M10.2 17 Origins of Autopolyploidy Nondisjunction of ALL chromosomes during meiosis produces diploid gametes (unreduced gametes). Diploid gamete + normal gamete = autotriploid (3n). Diploid gamete + Diploid gamete = autotetraploid (4n). M10.2 18 Effects of Autopolyploidy Triploid Usually sterile (oddnumbered ploidy). Possible Gametes Most gametes produced are genetically unbalanced. Triploid zygotes can produce many possible gametes. Each gamete can get either one or two copies of each chromosome, in many possible combinations. M10.2 19 Generating Allopolyploid Species M10.2 20 To convert sterile hybrid into fertile ‘new’ species, need chromosome doubling. o Hybrid is sterile. o Unbalanced gametes are nonviable. o But if entire genome is doubled by mitotic nondisjunction, the fertility problem is solved. M10.2 21 Significance of Polyploids: Agriculture Cell volume correlated with nucleus volume, correlated with genome size. Polyploids often have bigger leaves, fruits, seeds. Bread wheat is a polyploid derived from 3 species. See Table 8.2 – many of our most important crops are polyploids! M10.2 23 Significance of Polyploids: Agriculture Production of larger fruits, e.g. strawberries and grapes. Wild strawberries (2n=14) Production of seedless fruit (sterile), e.g. bananas, grapes and watermelon. Commercial strawberries (8n=56; allopolyploid), larger fruit. Wild diploid bananas (2n=22), lots of large seeds. M10.2 Commercial triploid bananas (3n=33, autopolyploid) are sterile. Cannot produce viable gametes undeveloped seeds. 24 Significance of Polyploids: Agriculture Production of larger fruits, e.g. strawberries and grapes. Wild strawberries (2n=14) Production of seedless fruit (sterile), e.g. bananas, grapes and watermelon. Commercial strawberries (8n=56; allopolyploid), larger fruit. Wild diploid bananas (2n=22), lots of large seeds. M10.2 Commercial triploid bananas (3n=33, autopolyploid) are sterile. Cannot produce viable gametes undeveloped seeds. But polyploidy is not always a good thing…our favourite bananas are in trouble Domestic bananas (mostly 3n = 33) are derived from 2 wild species: Musa acuminata (‘A’) and Musa balbisiana (‘B’). World production = 100 Mt ‘Gros Michel’, Cavendish are AAA 2n gametes from one species, 1n gamete from another Most plantains are ABB or AAB Most varieties derived from spontaneous hybrid polyploids found in the wild M10.2 Bananas in trouble… In 1950s & 1960s, Gros Michel wiped out by ‘Panama disease’ (Fusarium). Replaced by resistant Cavendish. In 1980s, a new strain of Fusarium, ‘Tropical Race 4’ appeared in Malaysia, now spreading around world. Cavendish has no resistance. Breeding Fusarium-resistant bananas that have desirable market qualities is not easy! M10.2 https://www.intrafish.com/aquaculture/study-triploid-farmed-salmon-virtually-sterile-containhigher-proportion-of-healthy-fats/2-1-443465 M10.2 Module 10.2 Variations in chromosome number: important concepts Difference between aneuploidy and polyploidy, and terminology associated with chromosome number variations. Origins of aneuploidy. Role of aneuploidy in some human genetic disorders, the importance of chromosome size, and the special case posed by sex chromosomes. How a genetic disorder such as Down Syndrome, usually caused by aneuploidy can also be caused by a Robertsonian translocation, and the implications for the transmission of the disorder from one generation to another. Why hybrids of related plant species are often sterile, and how & why nondisjunctions can restore fertility. Importance of polyploidy in formation of new plants species, and agricultural crops. M10.2 25 Extra slides: no new critical information – just a couple more examples of concepts introduced in the lecture. M10.2 Generating Allopolyploid Species 9R mitotic non-disjunction and self-fertilization. + 9B Raphanus sativus Brassica oleracea 18R 9R + 9B 18B 18R + 18B In 1928 geneticist Karpechenko hoped to produce a plant with the root of a radish and the head of a cabbage. Unfortunately, he got the opposite. Amphidiploid = allotetraploid Exhibit traits of both original species Cannot cross to either parent Effectively a new species – Raphanobrassica (aka ‘Cabbish’). M10.2 22 Published in Genome 51:113-119. DOI: 10.1139/G07-112 © Canadian Science Publishing or its licensors. © Éditions Sciences Canada ou ses concédants de licence. Another sturgeon species has ~437 chromosomes; some are functional heptaploids (7n) Karyotype of the shortnose sturgeon, Acipenser brevirostrum (2n = 374). The chromosomes are aligned according to decreasing size and are grouped into meta- and submetacentrics (178) and acrocentrics and microchromosomes (196). Magnification: × 1200. M10.2