26. Comparative development2

26. Comparative development2 - 3/30/11 3 Developmental...

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Unformatted text preview: 3/30/11 3 Developmental Biology Lectures 1.  General principles of animal development 2.  Hox genes in bilaterian development 3.  Compara=ve developmental biology To=potency vs. differen=a=on Symmetry – types and mechanisms Plant development 2) Totipotency vs. Differentiation T ­ the ability of a single cell to regenerate the en=re organism D ­ the acquisi=on of specialized structures and func=ons Some themes in the comparative development of multicellular eukaryotes 1. Enormous flexibility in bilaterian design  ­ ca. 30 modern phyla with dis=nc=ve body plans oJen u=lizing homologous molecular mechanisms 2.  Why study other mul=cellular eukaryotes? Fascina=ng examples of biological diversity Deep insights into the evolu=on and mechanisms of bilaterian development Several issues relevant to human medicine Totipotency vs. Differentiation Key feature – the segrega=on of reproduc=ve cells from the specialized soma (body cells) 1.  Prokaryo=c cells are poten=ally immortal 2.  Faus=an bargain of eukaryo=c reproduc=on ma=ng rituals of praying man=ses luna moths with no mouth parts 3.  Which cells are poten=ally immortal? Which aren’t? Volvox – mul=cellular green alga Dictyostelium – cellular slime mold 1 3/30/11 Animals – early segrega=on of germ line Totipotency vs. Differentiation Plants – late segrega=on of germ line (capable of meiosis) from persistent meristema=c (i.e., stem) cells – more later Zygote C. elegans embryo with p granules marking the germ ­line cell (P4) Germ ­line cells set aside early, but later migrate to the gonads Many fewer muta=ons accumulate in animal germ lines than plants Many mul=cellular eukaryotes express to=potency in cell culture Carrot Comparable examples of mammalian to=potency Dolly – first example of the genes from a differen=ated mammalian cell being capable of rever=ng to embryonic to=potent state See F. Fig. 35.13 2 3/30/11 Totipotency – Human stem cells Stem cells retain the ability to divide via mitosis and to differen=ate into various specialized cell types. Embryonic stem cells – to=potent or pluripotent Totipotency – Trouble in Paradise Somaclonal varia=on in plants – high levels of gene=c, chromosomal, and epigene=c abnormali=es in plants grown from cultured cells morula human fetus Adult stem cells – soma=c (body) or germline (producing gametes) h\p://en.wikipedia.org/wiki/Stem_cell Totipotency – Trouble in Paradise 1.Dolly’s premature death – 6 ­year ­old cells? 2. Shortened telomeres (chromosome ends)  ­ premature ageing? 3. Embryos from nuclear transfer expts – frequent developmental abnormali=es 4. Dolly – only 1 of 277 to make it to adulthood 5. Differen=a=on correlates – ageing, cancer, and death 6. Meiosis – repackaging chromosomes, removing epigene=c changes to genes and chromosomes, etc. 7. Next fron=er in human medicine Possible implica=ons for stem cell regenera=on? Any difference between adult vs. embryonic stem cells? 3) Design strategies of mul=cellular animals and plants One growth axis  ­ top to bo4om See F. Fig. 32.5 Two growth axes  ­ anterior to posterior (AP) dorsal to ventral (DV) Planes of symmetry = number of so ­called mirror images 3 3/30/11 Radial symmetry  ­ Cnidarians Animal version of modular development – sequen=al development of new axes C & R Fig. 33.4 One central growth axis  ­ top to bo\om Modularity  ­ Cnidarians Single polyp develops many individual feeding and reproduc=ve polyps Bilateral symmetry Flatworm Nematode Mollusc Hydra Aurelia (Class Scyphosa) Modularity  ­ the forma=on of new axes from the surface and/or new organisms from the central axis Annelid Arthropod Chordate Two growth axes  ­ anterior  ­ posterior (AP) axis and dorsal  ­ ventral (DV) axis CephalizaBon  ­ the separa=on of the AP axis into head region for feeding, sensa=on, and processing and tail region for locomo=on SegmentaBon  ­ the subdivision of AP axis into repea=ng segments in annelids, arthropods, and chordates 4 3/30/11 Evolution of developmental processes in basal metazoans Symmetry  ­ plants Pa\ern forma=on of developmental axes, plus induc=on Modular branching for organism forma=on Cell ­to ­cell recogni=on for organism assembly Animals  ­ phylogene=c switch from radial to bilateral symmetry Plants  ­ persistent radial symmetry of main and lateral axes, with repeated evolu=on of bilateral leaves Peterson and Davidson (2000) PNAS 97:4430 ­4433 Early land plants Most living plants  ­ aboveground structures Bryophytes Radial axes (stems)  ­ specialized for support and transport Bilateral organs (leaves)  ­ specialized for photosynthesis Vascular plants Silurian plants (400 mya) Apical meristem – growing =p of ~100 cells Early land plants  ­ “naked” cylindrical axes with no leaves Radial symmetry comes from hemispherical apical meristem that represents the growing =p of the axis Shoot apical meristem – generates new stem ­leaf modules 5 3/30/11 Basic principle of plant development Evolu=on of land plant symmetry RS  ­ radial symmetry MAM  ­ mul=cellular apical meristems L L  ­ bilateral leaves (at least four =mes) L L L MAM RS Gene=c regula=on of shoot apical meristems Background: Apical Meristems give rise to the en=re plant body: New cells for maintaining the meristem – stem cell func=on New cells for differen=a=ng organs Animals – determinate organisms – cell differentiation and organ formation occurs within defined body Plants – indeterminate organisms – new cells, organs, and meristems develop throughout the life span SAM = shoot apical meristem RAM = root apical meristem Gene=c regula=on of floral organs Model plant Arabidopsis C & R F of Polar view ig.35.35a floral meristem Wild ­type flower KNOTTED1-like homeobox genes (KNOX) control the transcription of other genes for maintaining meristematic state. Other genes, such as ASYMMETRIC LEAVES1 (AS1), suppress the expression of KNOX genes, resulting in the initiation of leaves. Polar view of F Fig. 22.11 mature flower Whorl 1 2 3 4 B genes A genes C genes Organ Se Pe St Ca Zhongchi Li (CBMG) ABC model for floral inducBon 6 3/30/11 Gene=c regula=on of floral organs Whorl 1 2 3 4 Summary  ­ ABC Model for Gene=c Regula=on of Floral Organs Three classes of MADS ­box genes act as transcrip=on factors for regula=ng floral development B genes A genes C genes Organ Se Pe St Ca A class by itself directs sepal forma=on A + B classes direct petal forma=on B + C classes direct stamen forma=on C class by itself direct carpel forma=on ABC model for floral inducBon C & R Fig.35.35a Whorl 1 2 3 4 AP3/PI AP1/AP2 AG Organ Se Pe St Ca Wild ­type flower C & R Fig.35.35b Can you use the ABC model to predict the phenotypes of several floral mutants? How would you test the model? C & R Fig. 35.36 A class = APETALLA1/APETALLA2 B class = APETALLA3/PISTILLATA C class = AGAMOUS ABC model for floral inducBon Compara=ve development of mul=cellular eukaryotes •  Mul=ple independent origins of eukaryo=c mul=cellularity •  To=potency and differen=a=on – alterna=ve expressions of cell specializa=on •  Segrega=on of the germ line from the soma •  The expression of to=potency in plants, Dolly, and human stem cells •  The challenge of maintaining the to=potency in cultured cells •  Molecular mechanisms for the development of unicellular pro=sts, animals, and plants •  Symmetry  ­ radial (1 growth axis) vs. bilateral (2 growth axes) in different organisms •  Plants  ­ radial symmetry in axes vs. bilateral symmetry in leaves •  Apical meristems and indeterminate growth in plants •  Gene=c regula=on of meristem ac=vity and flower forma=on •  ABC model for flower development 7 ...
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