Lecture Biology Chromosomal speciation

March 12, 2013 Speciation, Chromosomal speciation -no geographic barriers; -no exchange of genetic material between species > barriers occur at meiosis [post-zygotic] -definitions, ploidy, mitosis & meiosis (gamete formation) -homoploidy, ex.Helianthis, hybrid, new species -autoploidy & its advantages/disadvantages, -alloploidy & its advantages -summary of chromosomal speciation Summary of speciation processes Some contrasts between plant and animal development A B C D E Leaf Ovary Anthers or pollen Roots All of above A B C D E Eyes Ovary Testes Kidney All of above Determinate and indeterminate growth Some definitions, terms for chromosomes -chromosome number -ploidy (euploid, aneuploid) -Euploidy (whole sets of chromosomes) versus aneuploidy (partial sets of chromosomes) -how chromosome mutations differ from mutations at SNPs? -how they arise? A A n =n=? ? ?n = ?n = D n = ? D =n=? ?n 12 ?n =12 B Bn n=? = ? ?n = ?n =6 E n = ? E?nn ==? ?n = C n = ? C n=? ?n = ?n =9 F n = ? F ?n n ==? ?n = n =chromosomes in a haploid set ?n = number of copies (ploidy level) of each haploid set Shapes= chromosomes carry certain loci; sameshaped chromosomes are homologous Same-shaped chromosomes with different pattern carry different alleles A n = ? A n=? ?n = ?n = D n = ? D n =? ?n = 12 ?n =12 B n = ? B?nn=? = C n = ? ?n = C n?n== ?9 ?n = E n = ? E n = ?n = F n = ? F?nn= =8 ? ?n = ?n =8 Chromosome no A B C D E Ploidy level A 0.5 B C D E ? Chromosomal speciation models -Homoploidy (hybridization, chromosome rearrangement, no ploidy change) -Autoploidy (doubling own chromosomes) -Alloploidy (hybridization,yielding a haploid “zygote,” maybe some time and luck…then doubling chromosomes, yielding a new diploid species = new alloploid species) http://www.birkenholz.com/IMAGES/05MuleColtsHerd4.jpg Homoploidy: Hybridization can lead to new species ?? Problem: reproductive isolation? Fig 26.11 2- post-zygotic reproductive isolation; chromosome rearrangments (inversion) cause difficulties in crossing-over at meiosis (cause duplicate or missing loci) A A B B C C D D E E F F G G A A A A B B B B C F F F D E E E E D D D F C C C G G G G From Schwartzbach,Donovan, Rieseberg, 2001 Am J Bot.88(2): 270 -Key to homoploids’ success: 1- transgressive expression of traits more extreme than in either parental species Crossing over between chromosomes on left, or on the right is OK, but between in the middle (blue line) yield gametes with duplicate D loci and missing E loci—non-viable gametes! Identical? Or not? What is the chromosome number for these cells? Fig 12.8, text Autoploidy: doubling own chromosomes -causes “instant” speciation if plant can self-fertilize -does not introduce new genetic material; -tolerates mutations -isolated from parent diploid by problems at meiosis [post-zygotic] -evolutionary problem: lack of phenotype (expressed) variation; many heterozygotes, very few homozygotes Note chr coding by Color=homology in text [potentially misleading] Part of Fig 26.8 n + 2n = 3n Abnormal meiosis!! because odd number of homologous chromosomes leaves some unpaired Part of Fig 26.8 Part of Fig 26.8 Species Species Alloploidy: Two diploid parents from different species hydridize… Hybrid may be viable, vigorous, but is haploid Mitosis is normal Can’t Meiosis, because lack of homologous chromosomes A B C We’re now going to track individuals B and C… B: Normal meiosis in diploid organism: homologous chromosomes pair & duplicate, crossing over may occur, may exchange segments betw homologous chr (but note exchange with non-homologs causes missing or duplicated loci in some gametes) C: Sterile hybrid (it’s really a haploid) If chromosomes duplicate, it becomes functionally diploid; we call it ‘tetraploid’ to acknowledge its evol history Ex Tragopogon plants Plants 14 & 15 are diploid parents of alloploid sp 20 Plants 15 & 16 are diploid parents of alloploid sp 22 Source: M Ownbey 1950 Amer J Botany 37:487-499 Polyploids (mostly alloploids) in Arctic are very very common (69-87%) By comparison: % in West Africa ~26%, N Sahara ~38%, British Isles ~ 53% Sources: C Brochmann et all 2004 Biol J Linn Soc 82:521-536; Ricklefs, Ecology, 2nd ed p 397 Chromosomal speciation summary Chromosomal mutations can cause speciation via: -homoploidy: new hybrid combination isolated from parental species by chr rearrangement; hybrid sp has combination but also more extreme traits, transgressive expression -autoploidy: tolerates mutations (becomes heterozygote) but phenotypic variation reduced in pop – implications for long term evolution, esp natural selection? -alloploidy: combines genetic info from species has lots of genetic material of both in a diploid; but many may be unsuccessful haploids; if become alloploids often will outcompete both parental species Chr mut.“very” common: many chr muts.= new species? Allopatric Genetic diverg Repro Isol Key Factors Time req’d.(yr) [geog isolation, accumulate lots genetic differencesà post-zygygotic reproductive isolation, 2o contact, maybe character displaement] Vicariance lots post-zyg sel.,drift,mut ?~106 Colonization lots post-zyg drift,sel.,mut ?>103 Sympatric [no geographic isolation, no chromosome mututation, positive (+) assortative mating is essential à pre-mating repro isolation Sexual Sel little* Host shift little* Sp.mutualist v.little* pre-zyg pre-zyg pre-zyg sex.sel,+assort m ?fast disrup sel,+assort.m.