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MITOSIS THE HEREDITARY MATERIAL OF ORGANISMS (PLANTS) ARE ORDERED INTO CHROMOSOMES I. KARYOTYPE – THE CHARACTER TRAITS OF THE CHROMOSOMAL COMPLEMENT OF A CELL WITH REFERENCE TO THE COMPARATIVE SIZE, SHAPE, NUMBER AND MORPHOLOGY OF THE DIFFERENT CHROMOSOMES IN A CELL, INCLUDING THICKNESS, LENGTH, POSITION OF THE CENTROMERE, ETC. A. THESE TRAITS CAN DIFFER BETWEEN SPECIES B. MEASUREMENTS ARE DONE DURING EITHER MEIOSIS OR MITOSIS 1 C. WHAT IS THE DIFFERENCE BETWEEN MITOSIS AND MEIOSIS? WHAT STAGE IS THE BEST TO DETERMINE KARYOTYPIC TRAITS? II. CHROMOSOME NUMBER A. CHROMOSOMES ARE PRESENT IN PAIRS IN SEXUALLY REPRODUCING ORGANISMS (PLANTS) 1. THE TWO MEMBERS OF EACH PAIR ARE ESSENTIALLY IDENTICAL (KARYOTYPE) AND ARE CALLED HOMOLOGOUS CHROMOSOMES a. ONE HOMOLOGUE IS PATERNAL b. ONE HOMOLOGUE IS MATERNAL 2. THE TOTAL NUMBER OF CHROMOSOMES IN EACH CELL IS THE CHROMOSOME 2 COMPLEMENT 3. THE CHROMOSOME COMPLEMENT IS CONSTANT FOR ALL SOMATIC CELLS OF AN INDIVIDUAL PLANT B. HAPLOID = 1n = THE NUMBER OF CHOMOSOMES IN A GAMETE (EGG OR SPERM) AND IS CALLED THE GENOME C. DIPLOID = 2n = THE NUMBER OF CHROMOSOMES IN A ZYGOTE; HENCE THE NUMBER IN SOMATIC CELLS OF PLANTS. THEREFORE A DIPLOID HAS TWO CHROMOSOME SETS OR GENOMES III. POLYPLOID – COMPOSED OF THAN TWO GENOMES; TRIPLOID = 3x; TETRAPLOID = 4x; HEXAPLOID = 6x; OCTOPLOID =8x. 3 A. EUPLOIDY – THE GAIN OR LOSS OF COMPLETE SETS OF CHROMOSOMES OR GENOMES B. EUPLOIDY IS VERY COMMON IN PLANTS (RARE IN ANIMALS) AND CAN ARISE IN THREE BASIC WAYS: 1. SOMETHING CAUSES A DOUBLING OF THE CHROMOSOME NUMBER IN A CELL – THIS USUALLY HAPPENS IN A SOMATIC CELL, AND WHEN IT HAPPENS IN THE MERISTEM THE PLANT CAN BE AFFECTED (X-RAYS, UV LIGHT, CHEMICALS, ETC.) 2. SOMETHING GOES WRONG IN MEIOSIS, RATHER THAN REDUCING THE NUMBER OF CHROMOSOMES TO A 1n GAMETE, GAMETES ARE NOW 2n 4 3. TWO PLANTS WITH DIFFERENT GENOMES HYBRIDIZE – AA X BB = AB a. NORMALLY THE F1 HYBRID IS HIGHLY STERILE BECAUSE OF THE INABILITY OF NON-HOMOLOGOUS CHROMOSOMES TO PAIR IN MEIOSIS – A AND B b. IN NATURAL POPULATIONS STERILE HYBRIDS ARE OFTEN COMMON c. HOWEVER, IF THERE IS A DOUBLING OF THE CHROMOSOME NUMBER TO GET AABB NOW NORMAL PAIRING IN MEIOSIS CAN OCCUR AND THE PLANT IS FERTILE IV. ANEUPLOIDY – THE GAIN OR LOSS ON ONE OR MORE CHROMOSOMES OF A SET – NO LONGER EXACT MULTIPLES OF THE BASIC CHROMOSOME NUMBER 5 A. THIS IS USUALLY CAUSED BY NONDISJUNCTION – THE FAILURE OF CHROMOSOMES IN A PAIR TO SEPARATE IN THE MEITOIC PROCESS RESULTING IN GAMETES WITH EITHER MORE OR FEWER CHROMOSOMES B. IN DIPLOIDS THIS IS USUALLY LETHAL, BUT IN POLYPLOIDS, BECAUSE OF THE NUMEROUS DUPLICATIONS OF CHROMOSOMES, IT IS USUALLY SURVIVABLE BY THE PLANT V. EFFECTS OF POLYPLOIDY A. CHANGE IN CHROMOSOME NUMBER CHANGES GENETIC SEGREGATION RESULTING IN NEW GENETIC COMBINATIONS B. ANY INCREASE IN CHROMOSOME NUMBER PROVIDES A MASK OR COVER FOR 6 DELETERIOUS RECESSIVE GENES, SO ANY LETHAL GENES ALREADY IN THE GENOME CAN BE HIDDEN C. LOSS OF AN ENTIRE CHROMOSOME (ANEUPLOIDY) OR SEGMENTS OF A CHROMOSOME, CAN OCCUR WITHOUT PHENOTYPIC EFFECT BECAUSE OF THE DUPLICATION OF OTHER CHROMOSOMES CARRYING THE SAME GENES D. AN INCREASE IN DELETERIOUS MUTATIONS (AN MOST ARE) CAN INCREASE GREATLY IN A POLYPLOID BECAUSE OF REDUNDANCY E. AN INCREASE IN CHROMOSOME NUMBER IS USUALLY ACCOMPANIED BY AN INCREASE IN CELL SIZE = LARGER CELLS 7 THIS IS A PARTIAL REASON FOR THE SUPERIOR AGRICULTURAL PERFORMANCE OF MANY POLYPLOIDS – BIGGER CELLS – BIGGER PLANTS – BIGGER FRUITS AND SEEDS F. GAMETE STERILITY AND LOWERED FECUNDITY CAN ACCOMPANY POLYPLOIDY G. POLYPLOIDS OFTEN HAVE THE ABILITY TO EXPLOIT OR OCCUPY NEW AND DIVERSE ECOLOGICAL HABITATS – ESPECIALLY SITES THAT ARE DISTURBED H. SPECIATION – POLYPLOIDY IS ONE WAY NEW SPECIES EVOLVE – WHEN TWO DIFFERENT SPECIES ARE INVOLVED IN THE ORIGINAL CROSS. 8 POLYPLOIDY DOES NOT ALWAYS RESULT IN SPECIATION, ESPECIALLY WHEN ONE SPECIES MAY BE INVOLVED IN A CROSS OR DOUBLING. THERE EXISTS INTRASPECIFIC NONHYBRID POLYPLOIDY. THIS INVOLVES MULTIPLICATION OF THE SAME CHROMOSOMAL SET = AUTOPLOIDY. IT MAY OR MAY NOT LEAD TO SPECIATION. (FESTUCA) POLYPLOIDY ASSOCIATED WITH INTERSPECIFIC HYBRIDIZATION RESULTS IN THE DOUBLING OF STRUCTURALLY DISSIMILAR CHROMOSOMES IN A SPECIES HYBRID = ALLOPLOIDY. (TRITICUM) VI. POLYPLOIDY & ANEUPLOIDY ARE CONSPICUOUS FEATURES IN THE EVOLUTION 9 OF PLANTS. ESTIMATES OF THE NUMBER OF SPECIES THAT ARE POLYPLOID ARE: ANGIOSPERMS 47% DICOTS 43% MONOCOTS 58% GYMNOSPERMS 1.5% PTERIDOPHYTES 95% OF GREAT INTEREST IS THE BASIC CHROMOSOME NUMBER – THE HAPLOID NUMBER OF THE DIPLOID SPECIES OF A POLYPLOID SERIES. ALL CHROMOSOME NUMBERS IN A POLYPLOID SERIES ARE DIVISIBLE BY THE BASIC NUMBER. FESTUCA 2n = 14 n=7 diploid 2n = 28 n=14 tetraploid 10 2n = 42 n=21 hexaploid 2n = 56 n=28 octoploid 2n = 70 n=35 decaploid THE BASIC CHROMOSOME NUMBER IS X=7 THIS IS AN EXAMPLE OF A POLYPLOID SERIES. CHROMOSOME NUMBERS IN THE ANGIOSPERMS RANGE FROM 2n=4 (HAPLOPAPPUS GRACILIS, ZINGERIA) TO 2n= CA. 500 (KALANCHOE). IN THE FERNS THE NUMBER OF CHROMOSOMES PER CELL REACHES ITS HIGHEST NUMBER IN OPHIOGLOSSUM RETICULATUM WITH 2n=1260. 11 12