Mendelian Genetics and Plant Sexual Reproduction

Mendelian Genetics and Plant Sexual Reproduction R. N. Trigiano, Professor EPP 15 January 2014

Alternation of Generations • Mitosis and Meiosis • Mendelian Genetics • Sexual Reproduction in Plants

ALTERNATION OF GENERATIONS DEFINITIONS: Diploid: Having two sets of chromomosomes or 2N – the sporophyte phase of plants. 2N = 2X = 22 (Flowering dogwood) Haploid: Having one set of chromosomes or N – the gametophyte phase of plants. N = X=11 (Flowering dogwood) Polyploid: Having more than two sets of chromosomes – 2N = 6X = 54 sporophyte; gametophyte N = 3X= 27 (Chrysanthemum)

How is Alternation of Generations Accomplished? Mitosis (1 cell 2 cells) and Meiosis (1 cell 4 cells) and Sexual Reproduction where 1 +1 = 2 = 1! Gametophyte (N) mitosis Gametes (N) fuse (sexual reproduction) to form zygote (2N) mitosis Mature Sporophyte (2N) meiosis Spores (n)

ALTERNATION OF GENERATIONS Lower Plants Higher Plants SHADED AREA REPRESENT RELATIVE TIME IN DIPLOID OR SPOROPHYTIC PHASE; OPEN = RELATIVE TIME IN HAPLOID OR GAMETOPHYTIC PHASE.

1 MOSS FERN A www.calflora.net silviahartmann.com D E A B D C C B

Mitosis and Meiosis http://www.vcbio.science.ru.nl/images/cellcycle/mmitosis_onion_zoom.jpg http://www.phschool.com/science/biology_place/biocoach/index.html

Figure 6.1 Structure of DNA Nitrogenous bases Adenine Thymine Guanine Cytosine Phosphate (PO4) Deoxyribose sugar

Figure 2.2 A. Chromosome Physical States Nucleus Nucleus Relaxed Chromosome Condensed Chromosome B. Chromosome Conformations C. Homologous Chromosomes Chromatid Sister Chromatids DNA Synthesis Locus 2 Centromere Centromere Locus 1 Chromosome 1 Chromosome 2 Chromosome 1 Chromosome 1

http://nobelprize.org/nobel_prizes/medicine/laureates/2001/cellcycle_eng.jpghttp://nobelprize.org/nobel_prizes/medicine/laureates/2001/cellcycle_eng.jpg

STAGES OF MITOSIS PROPHASE: Shortening and thickening of chromosomes (actually sister chromatids: Movement to the metaphase plate; nucleous disappears; nuclear membrane disappears. METAPHASE: Chromosomes lie on the “ cell plate”. ANAPHASE: Sister chromatids separate and are pulled toward the poles. TELOPHASE: Last phase of mitosis; chromosomes become reorganized into nuclei, nucleolus reappears and nuclear membrane is reformed. CYTOKINESIS: Literally division of the cytoplasm (including organelles such mito- chondria, chloroplasts, etc. Essentially, mitosis is “asexual division (formation of somatic cells and builds the plants). Theoretically, all cells resulting from mitosis are “clones” – not quite true, but close enough for our purposes.

http://tainano.com/Molecular%20Biology%20Glossary.files/image011.gifhttp://tainano.com/Molecular%20Biology%20Glossary.files/image011.gif

1 = Interphase, 2-5 = Prophase; 6 = Early Metaphase; 7 = Late Metaphase; 8 = Anaphase; 9 = Telophase; 10 = Completion of Cytokinesis http://www.vcbio.science.ru.nl/images/cellcycle/mmitosis_onion_zoom.jpg

OH NO! MORE DEFINITIONS Meiosis: Two successive divisions (reduction followed by an equal) that reduces the ploidy level from 2n to n or diploid to haploid. Homologous Chromosomes: Chromosomes that associate in pairs in the first division of meiosis: each member of the pair was derived from a different parent. Crossing Over: The exchange of corresponding segments of genetic material between chromatids of homologous chromosomes. See also Synapse and Chiasma Recombination: The process by which offspring can gain combination of genes different from the combinations in either of their parents. Leads to genetic diversity.

Figure 2.9. Crossing- over B. C. A. Synapse – pairing of homologous Chromosomes Chiasma - Exchange between homologous chromosomes. Actually sister chromatids during Prophase I. Results in recombination and increased diversity

Stages of Meiosis Prophase: Same as in mitosis except that crossing over occurs here. Metaphase I: Homologous chromosomes (each chromosome consists of 2 sister chromatids) line up “opposite” each other on the plate. Anaphase I: One of each of the homologous pair of chromosomes are “drawn” to the poles. This essentially reduces the number of chromosomes that are in each cell by half. Diploid Haploid. Telophase I: Haploid cells are established; chromosomes “disappear”. Prophase II: Chromosome condense (sister chromatids are not identical to one another because of crossing over). Metaphase II: Chromosomes (actually sister chromatids) align on the plate. Anaphase II: One sister chromatid (now chromosome) are drawn to each pole. Teleophase II and Cytokinesis: Essentially the same as in mitosis. Meiosis is essentially a “reduction” division followed by an “equal” division. “Tetrad” (4) of cells or spores are formed.

Figure 2.8. Mitosis Meiosis I Meiosis II Metaphase Metaphase I Metaphase II 2N N 2N Metaphase Plate Metaphase Plate Metaphase Plate Anaphase I Anaphase II Anaphase Cytokinesis Cytokinesis Cytokinesis N N N

Figure 2. 3. Gregor Mendel, working with peas, made two innovations to the science of genetics: 1) developed pure lines 2) counted his results and kept statistical notes

SOME MORE DEFINITIONS Pure Line - a population that breeds true for a particular trait [this was an important innovation because any non-pure (segregating) generation would and did confuse the results of genetic experiments] Allele - one alternative form of a given allelic pair; tall and dwarf are the alleles for the height of a pea plant; more than two alleles can exist for any specific gene, but only two of them will be found within any individual Dominant - the allele that expresses itself at the expense of an alternate allele; the phenotype that is expressed in the F1 generation from the cross of two pure lines. Recessive - an allele whose expression is suppressed in the presence of a dominant allele; the phenotype that disappears in the F1 generation from the cross of two pure lines and reappears in the F2 generation Co-dominance: a relationship among alleles where both alleles contribute to the phenotype of the heterozygote. Allelic pair - the combination of two alleles that comprise the gene pair http://www.ndsu.edu/instruct/mcclean/plsc431/mendel/mendel1.htm

More Definitions !!! • Homozygote - an individual which contains only one allele at the allelic pair; for example DD is homozygous dominant and dd is homozygous recessive; pure lines are homozygous for the gene of interest • Heterozygote - an individual which contains one of each member of the gene pair; for example the Dd heterozygote • Genotype - the specific allelic combination for a certain gene or set of genes • Phenotype - literally means "the form that is shown"; it is the outward, physical appearance of a particular trait • Mendel's pea plants exhibited the following phenotypes: • - round or wrinkled seed phenotype • - yellow or green seed phenotype • - red or white flower phenotype • - tall or dwarf plant phenotype http://www.ndsu.edu/instruct/mcclean/plsc431/mendel/mendel1.htm

Mendel’s Laws Mendel's First Law - the law of segregation; during gamete formation each member of the allelic pair separates from the other member to form the genetic constitution of the gamete e.g. A a yields gametes with Aa Mendel’s Second Law -- the law of independent assortment; during gamete formation the segregation of the alleles of one allelic pair is independent of the segregation of the alleles of another allelic pair (assume no close linkage)

Figure 2.5. Mendel's First Law - the law of segregation; during gamete formation each member of the allelic pair separates from the other member to form the genetic constitution of the gamete Parent 1 Parent 1 Y y Y Y YY Yy Yy Yy Y y Parent 2 Parent 2 Yy yy Y Yy Yy y A. Monohybrid Cross B. F1 Self Fertilization Parent 1 Parent 2 Parent 2 Parent 1 X X Yy YY yy Yy Gametes: Y Y y y Gametes: Y y Y y F1 Fertilization: F2 Fertilization: YY & Yy Yy F2 Plants: 75% yellow 25% green F1 Hybrid Plants: 100% yellow yy

ty ty TY TtYy TtYy TtYy TtYy TY F1 Generation of Dihybid cross

Figure 2.6. Independent Assortment – Two or more traits (color and height). Parent 1 TY Ty tY ty TTYY TTYy TtYY TtYy TY TTYy TTyy TtYy Ttyy Ty Parent 2 TyYY TtYy ttYY ttYy tY TtYy Ttyy ttYy ttyy ty A. Dihybrid Cross Parent 1 Parent 2 TTYY X ttyy F1 Fertilization: TtYy F1 Plants: 100% tall/yellow B. F1 Self Fertilization TtYy X TtYy Ratios of F2 Plants: F2 Fertilization: T_Y_ = 9/16 tall/yellow T_yy = 3/16 tall/green ttY_ = 3/16 short/yellow ttyy = 1/16 short/green

Co-dominance –Variation to Mendel “laws” Another Monohybrid Cross RR = Red Flower Parent -- rr = White Flower Parent r r Rr Rr R F1 R Rr Rr If Dominance (R) then all should be RED

r r But co-dominance of R and r produces all pink flowers R Rr Rr Rr Rr R F1 r R Rr R RR F2 1 Red; 2 Pink; 1 White rR rr r

Flowers and Sexual Reproduction

5 Flower Parts – Only Angiosperms! PIST ST SEP PET Figure 36.5. Morphology of a dicot flower. The perfect flower of this Oxalis species has five sepals (SEP), which are hidden from view in the open flowers by the five petals (PET), five stamens (ST) and one pistil (PIS). Compare to Figures 36.5 and 36.6.

4 ST AN ST PIST STL FL OVY PET OVL SEP REC RAF

6 STY SEP OVY OVL PET LOC OVL OVL FL OVY FIL CAR PET SEP REC B A

8 T MMC B C A GN S PTN PTN PT D E RAF

7 A V I MSMC M B HN N C DN FMSN D AN PN E M S RAF

You’re Right!!! More Definitions Self Incompatibility: Plant that cannot fertilize its female gametes with its own male gametes. Basically two types: Gametophytic in which the pollen grain will not grow when it shares the same incompatibility gene (s) and Sporophytic in which the diploid parent hinders germination and growth of the pollen tube. Both types prevent double fertilization. Outcrossing Species: basically self-incompatible; requires a partner of a different genotype. Pollination: The process in which the pollen is carried from the anther to the stigma.Basically two types: Cross (between different genotypes of plants) and Self (same plant). Double Fertilization: one sperm nucleus unites with egg to form the zygote and restore the sporophytic phase (2N) of the plant and the other sperm nucleus uniting with the polar nuclei of the female gametophyte to form the primary endosperm nucleus.

Figure 2.12 Gametophytic Incompatibility Sporophytic Incompatibility

Double Fertilization 9 AN PN E SP PT B N PG PN 2 1 E PT PT C OVY PEN ZY OVL D A RAF

Zygotic Embryo Ontogeny -- Dicot Figure 4.4

10 YGE I EN E S Zygotic embryogenesis Cercis canadensis (Eastern Redbud) A A D GE I EN S C B C SM H VT RM C E

11 Somatic Embryogenesis EN SC CL SM Orchardgrass – a monocot SN RM CR A B EN C I SM C Eastern Redbud – a dicot H SM H C D

Summary Slide 1 • All plants (and animals) exhibit alternation of generations • For higher plants, the sporophytic (2N) generation is the dominant dominant phase • Meiosis is the driving biological process behind alternation of generations.

Summary Slide 2 • Mitosis in an equal division in which the ploidy of the mother cell is maintained • The phases of mitosis are Prophase, Metaphase, Anaphase, and Telophase followed by Cytokinesis. • Meiosis is a reduction division followed by an equal division similar to mitosis. Results in four cells of reduced (haploid) ploidy • Crossing over (exchange of sister chromatid segments occurs in Prophase I and results in genetic recombination.

Summary Slide 3 • Mendel's First Law - the law of segregation; during gamete formation each member of the allelic pair separates from the other member to form the genetic constitution of the gamete • Mendel’s Second Law -- the law of independent assortment; during gamete formation the segregation of the alleles of one allelic pair is independent of the segregation of the alleles of another allelic pair (assumes no close linkage) • Expression of alleles can be dominant, recessive or codominant.

Summary Slide 4 • The four floral organs are pistils, anthers, • sepals and petals. • Higher plants do NOT require water for fertilization to occur • Male gametophyte is 3 cells, whereas the female gametophyte is 8 cells (nuclei) • The process of double fertilization in higher plants produces a zygote (2N) and primary endosperm nucleus (3 or 5 N)

Mendelian Genetics and Plant Sexual Reproduction