Ploidy – Part 1: An Overview PBG 430

Ploidy – so many options Ploidy = number of chromosome copies in a genome Haploid Diploid Polyploid Haploid Triploid Autotetraploid Allohexaploid Allopolyploid with variable chromosome number

Terms to Remember: Euploidy Total chromosome number is an exact multiple of a basic chromosome number (x) Can be diploid (2x), triploid (3x), tetraploid (4x), ….. Examples Barley (diploid): 2n = 2x = 14 Base number (x) = 7 Gametophytic generation (n) = 7 Sporophytic generation (2n) = 14 Durum wheat (tetraploid): 2n = 4x = 28 Gametophytic generation (n) = 14 Sporophytic generation (2n) = 28

Terms to Remember: Aneuploidy Not euploid  more or fewer chromosomes than a multiple of the basic number Classifications “Monosomic” = loss of a chromosome, (2n-1) “Trisomic” = addition of a chromosome, (2n+1) Aneuploids have great value in genetic studies (e.g. locating genes in chromosomes)

Haploid barley growing in the lab Haploidy Typically, haploid angiosperm plants will be sterile Diploid (fertile) Haploid barley growing in the lab Haploid (sterile)

Polyploidy Polyploids have more than two basic sets of chromosomes (3X, 4X, 6X, etc.) Autopolyploidy Chromosome sets originate from the same species Result of genome doubling within a species Allopolyploidy Chromosome sets originate from different ancestral genomes Result of interspecific hybridization and genome doubling

Polyploids – Mitosis and Meiosis Mitosis = ok Meiosis ? Bivalent pairing: essential for the formation of reduced (n) and balanced gametes

Autopolyploids Even number of chromosomes (4x, 6x, etc.) Plants can be fertile…if there is bivalent pairing & normal meiosis Examples: Potato  2n = 4x = 48 Alfalfa  2n = 4x = 32 Odd number of chromosomes (3x, 5x, etc.) Plants are usually sterile or abnormal Because the complete chromosome complement cannot form into pairs, which causes normal meiosis to be disrupted Aneuploidy Example: Banana  2n = 3x = 33

Autopolyploidy – Comparison Example Characteristic Diploid Example Tetraploid Example Genome Quantity 1 genome = Genome A 2 genomes = Genome A + duplicated Genome A Locus Quantity 1 locus = V locus 2 loci = V locus + duplicated V locus Gene Quantity 1 gene = V gene 2 genes = V gene + duplicated V gene Allele Quantity 2 alleles = AA 4 alleles = AAAA Homologous Chromosomes Quantity 1 pair of homologous chromosomes = AA 2 pairs of homologous chromosomes = AAAA versus

Allopolyploids An individual with chromosome sets from two or more different, but related, species Interspecific hybridization + genome doubling: Unreduced gametes Reduced gametes (normal fertilization) but abnormal mitosis Genome doubling can be either spontaneous (natural forms – bread wheat) or artificial (colchicine – doubled haploids) Usually fertile  bivalent pairing (behave like diploids) Homologous chromosomes within each ancestral species pair

Allopolyploidy – Comparison Example Homologous vs. Homoeologous chromosomes? Characteristic Diploid Example Tetraploid Example Hexaploid Example Genome Quantity 1 genome = Genome A 2 genomes = Genome A + Genome B 3 genomes = Genome A + Genome B + Genome C Locus Quantity 1 locus = V locus 2 loci = V locus (A) + V locus (B) 3 loci = V locus (A) + V locus (B) + V locus (C) Gene Quantity 1 gene = V gene 2 genes = V gene (A) + V gene (B) 3 genes = V gene (A) + V gene (B) + V gene (C) Allele Quantity 2 alleles = AA 4 alleles = AABB 6 alleles = AABBCC Homologous Chromosomes Quantity 1 pair of homologous chromosomes = AA 2 pairs of homologous chromosomes = AA, BB 3 pairs of homologous chromosomes = AA, BB, CC Homoeologous Chromosome Quantity None 2 sets of homoeologous chromosomes = AA/BB 3 sets of homoeologous chromosomes = AA/BB/CC

Polyploidy – Pros and Cons Advantages Disadvantages Hybrid vigor: More alleles, more loci, more genes = potential for heterozygosity Allopolyploids = fixing of divergent parental genomes Redundancy – masking of recessive alleles Buffering capacity – differential gene expression Changes in cell structure & shape – doubling genome content may increase cell volume Sterility (assuming that’s a goal) Changes in cell structure & shape – doubling genome content increases cell volume Example: brittle wood in timber species Sterility – if fertility is the goal

Genetic tables for example species – Autopolyploids Plant Banana (Musa spp.) Formula 2n = 3x = 33 Genome size 472 Mb Approximate number of genes 36,000 Genome sequence Sequenced! (See Canvas Supplemental materials for more info) Pollination biology Asexual (stalks), monoecious imperfect flowers (infertile) Center of origin Malaysia Plant Potato (Solanum tuberosum) Formula 2n = 4x = 48 Genome size 844 Mb Approximate number of genes 39,000 Genome sequence Sequenced! (See Canvas Supplemental materials for more info) Pollination biology Asexual (tubers), hermaphroditic selfing Center of origin South America (Andes)

Genetic tables for example species – Allopolyploids Plant Wheat (Triticum aestivum) Formula 2n = 6x = 42 Genome size 13,916 Mb Approximate number of genes 40,000 Genome sequence Sequenced! (See Canvas Supplemental materials) Pollination biology Hermaphroditic selfing Center of origin Mediterranean and Southeast Asia

By now you should be able to… Define the many terms we learned in this lesson (remember “n” is used for generation cycle and “x” is used for ploidy level): Euploidy (2n+0) Aneuploidy (2n-1, 2n+1, …) Polyploidy (3x, 4x, 5x, …) Allopolyploid vs. autopolyploid Hypothesize if a plant is fertile or sterile, based on its ploidy level and type (allopolyploid vs. autopolyploid). Why are bananas sterile while wheat is not? What is the genetic mechanism (in meiosis) that is responsible for this difference? Differentiate between homologous chromosomes and homoeologous chromosomes in an allopolyploid. Estimate the number of genes, loci, and alleles in an induced autopolyploid as compared to the diploid from which it was induced. Discuss advantages and disadvantages of polyploidy in natural ecosystems and agroecosystems.