Development and Evolution

1. Development and Evolution

2. Just a few helpful terms • epigenetic control – control of development that occurs through the products of genes other than the gene(s) which actually are responsible for formation of the structure itself. • heterochrony – change due to change in timing • heterotopy – change in the position at which a character is expressed.(spatial) (exp. on thorax rather than abdomen) • heterometry – a change in quantity or degree of gene expression. (exp. two pairs of wings instead of one) • heterotypy- phenotypic change from one type to another (exp. walking legs to swimming legs)

3. How many gene changes are needed to account for the diversity of forms seen in the animal and plant kingdoms? Development and Evolution -Today

4. Not as many as you might think • New discoveries are providing clues to the answer • Research shows that there are often very few and small genetic differences between species that exhibit verydifferent adult forms. (example humans, apes and chimps) • similar genetic and cellular mechanisms underlie the development of embryos in species whose adult forms are very different

5. How many gene changes are needed to account for the diversity of forms seen in the animal and plant kingdoms (cont) • It is now possible to identify small genetic changes which are responsible for large phenotypic variations • A history of these changes is also being pursued through phylogenetic analysis

6. Homeotic genes • A class of genes that code for proteins that bind to DNA and regulate the expression of a wide range of other genes. • The actual binding capability resides in a particular location of the regulatory protein called the homeodomain. • The homeobox is a nucleotide sequence that codes for this homeodomain • The homeobox is very similar in many eukaryotic organisms and is about 180 base pairs.

7. Homeotic genes • Homeotic gene products provide positional information in a multicellular embryo. • They are involved in evolutionary change when.... • New features of multicellular organisms arise due to manipulation of pre-existing cell types. For example when… • the same cells arrive at new locations (heterotopy) OR • the same cells are expressed at different times of development (heterochrony)

8. Specification of a Cell’s Fate Is a Major Evolutionary Mechanism for Production of Different Organismal Forms • Multicellular organisms need a system for arranging cells in 3 dimensional space to assure … • proper organization of symmetry, segmentation, and body cavities • that cleavage and gastrulation occur correctly • correct differentiation of tissues (such as nervous, muscle, gut etc.)

9. Genes collectively referred to as Hox genes Specification of a Cell’s Fate Is a Major Evolutionary Mechanism for Production of Different Organismal Forms (cont) • Cells need to be identified based on their location in relation to … • other cells • time of expression • Genes that carry information for this control are called homeotic loci • In animals these loci are called • HOM loci in invertebrates • Hox loci in the vertebrates • Study of these genes is a new and hot area of research

10. Hox genes • Found in all major animal phyla • organized in gene complexes i.e. they are found in close proximity to each other on the chromosome • Appear to be the result of duplication events • each taxa surveyed shows unique patterns of duplication or loss in these loci

11. Hox Genes (cont) • Hox genes have temporal and spatial colinearity which is unique to HOM and Hox genes • Have perfect correlation between the order of genes along the chromosome and the anterior-posterior location of their gene products in the embryo. • Genes at the 3’ end are expressed in the head region and genes at the 5’ end in the posterior part of the embryo • Also 3’ genes are expressed earlier in development than those located toward the 5’ end • Finally the 3’ end produces greater quantity of product. • Called spatial, temporal and quantitative colinearity. This is unique to Hox sequences.

12. Hox Genes (cont) • Hox Genes regulate the location of appendages on body segments. • Work in embryo to specify the location where appendages should be located • Also determine whenin embryonic development these structures will be formed • Hox genes do not control the actual formation of appendages • Other genes control the formation of the actual structure whether a wing, leg, antenna, etc • However, these other genes are thought to be regulated by the regulatory protein products of Hox loci

13. When did the HOM/Hox loci originate? • These genes are found in not only segmented animals but also in… • plants • fungi • roundworms • other non-segmented animals • sponges!

14. When did the HOM/Hox loci originate? (cont) • Origin may have accompanied the development of multicellularity • Predates the development of a differentiated body axis • Therefore these genes also influence processes other than the specification of anterior to posterior cell fates

15. How do changes in HOM/Hox gene clusters lead to phenotypic changes? • Use of phylogenetic mapping has helped determine which genes in the Hox complex have been gained or lost at key branching points in the tree • Each major clade in the bilateral animals is characterized by a particular suite of Hox genes

16. How do changes in HOM/Hox gene clusters lead to phenotypic changes? • Examples • addition of the locus called Abdominal-B is associated with the evolution of bilateral animals • duplication of the entire Hox complex several times leads to the mouse and other vertebrates

17. Puzzles arising from the study of Hox genes • In the lower animals (sponges and cnidarians) there is a correlation between the number of HOM loci and the complexity of the animal’s body plan • The most primitive sponges and Cnidarians have just 5 loci • Sea urchins have 10 • mice have 39

18. Puzzles arising from the study of Hox genes (cont.) • This would support the idea that increase and elaboration of the number of Hox genes helped make the Cambrian “explosion” possible • However, in the Bilateria, this trend does not hold up and there is no real correlation between morphological complexity and numberofHoxloci • Most of the post-Cambrian diversification is due to changes in the timing or spatial location of Hoxgene expression rather than inthe number of the loci present

19. Arthropod segmentation and HOM/Hox expression • Initial diversification of the Arthropods occurred in the Cambrian, with even more diversification occurring much later. • Diversification of Arthropods relies heavily on differentiation of body segments from posterior to anterior • This differentiation is due to Hox gene expression

20. Arthropod segmentation and HOM/Hox expression (cont) • Still, all of the arthropod taxa have the same complement of 9 Hox genes • Morphological diversification is due to changes in localized expression of genes rather than the addition of new HOM loci • The genes which control morphological diversification are activated or suppressed by the action of a large number of HOM gene products

21. Arthropod segmentation and HOM/Hox expression (cont) • In turn the Hox genes influence the expression of a large number of other genes and developmental processes • Much of the evolutionary diversification of the arthropods probably occurred as a result of changes in whereHoxgenes are expressed • Fig 19.4the arthropod groups • Fig 19.5

22. The genetics of homologylimbs as an example

23. The tetrapod limb • Tetrapod limbs are a variation on a theme • Found on birds, mammals, reptiles, amphibians • First developed to allow mobility on land and then have undergone enormous variation

24. From whence came the limb? • sister group of tetrapods is the Panderichthyidae • lobe-finned large predators in shallow fresh water habitats • There are structural homologies between the groups • Fig 17.7 Member of the Panderichthyidae is the recently discovered Tiktaalik roseae

25. What genetic changes led to the tetrapod limb? • All tetrapods show the same basic features of limb development • A bud forms from mesodermal cellsFig 19.8a • At the tip of the bud is the AER (apical ectodermal ridge) • AER secretes molecules that keep cells in an undifferentiated state, this area is called the progress zone • The progress zone grows outward and defines the long axis of limb development • Fig 19.8 b and c

26. What genetic changes led to the tetrapod limb? (cont) • At the base of the bud is a group of cells, ZPA (zone of polarizing activity). • molecules secreted from the ZPA diffuse into the surrounding tissue and establish a gradient that supplies positional information to cells in the structure • the concentration of the molecule is critical and controls the timing and the 3-dimensional spacing of cell types

27. What genetic changes led to the tetrapod limb? (cont) • Spatial dimensions are defined in relation to the main body orientation • From anterior to posterior (thumb to little finger) • dorsal to ventral (back of hand to palm) • proximal to distal (arm to fingertips)

28. What genetic changes led to the tetrapod limb? (cont) • The concentrations of the molecules coming from the AER and ZPA coordinate the system and tell the cells where they are in 4 dimensional space ( time plus 3 dimensional space) • The molecules that control the 3 spatial coordinates are known (shh, Wnt7a, and Fgf-2 ) the ones that control the timing (temporal element) are not yet known • Hox genes are responsible for telling the cells where they are along the limb and control development of limb parts

29. Homologous genes and developmental pathways underlie the structural homology of tetrapod limbs • Adaptive changes could be due to changes in the timing ofor level of expression of pattern-forming genes (shh, Fgf-2 or Wnt7a) or the Hox complex • In fact it has been demonstrated that tetrapods have gained hands and feet as a result of a change in timing and location of homeotic gene expression

30. HHMI video clips • Lecture 3 chapter 28 36 min  50 min • Lecture 4 chapter 10 12 min  24 min

31. Flowers • Made the transition to land in the Silurian • there have been four major radiations • Rhyniophyta –first land plants • Ferns –first vascular tissue for conducting water • Early seed plants without flowers – seeds, pollen and spores freed them from their need for water during reproduction • angiosperms – floral diversity and association with insects allows wider distribution.

32. Flower development • Figure 19.15 parts of the flower • Flower morphology is dependent on homeotic genes • specify which organs appear in which locations • have DNA-binding regions called MADS analogous to homeobox of the Hox loci

33. Flower development (cont) • Have identified three types of mutants based on various homeotic mutations • Class A Fig 19.16 a • Class B Fig 19.16 b • Class C Fig 19.16 c • Combinations of mutations lead to the replacement of floral organs with leaf-like structures Fig 19.17 • Act very much like homeotic mutants in animals where one limb type can be replaced by another or limb differentiation can be prevented altogether

34. Four Homeotic Floral Genes • AP1 ( APETALA1) – expressed early in floral development controls formation of outer two whorls, sepals and petals. (Class A) Failure of this gene leads to no sepals or petals and a class A mutant

35. Four Homeotic Floral Genes • AP3 (APETALA3) – expressed later and involved in middle whorls of the flower bud. (Class B) Failure of this gene leads to lack of stamens and petals and a Class B mutant

36. Four Homeotic Floral Genes • AG (AGAMOUS) – expressed late and involves the center of the flower bud. (Class C) Failure of the AG gene leads to loss of stamens and carpels and a class C mutant

37. Four Homeotic Floral Genes • LFY (LEAFY) is a master control gene on which the other 3 are dependent • The LFY protein activates AP1, AP3 and AG expression in the appropriate cells of the four whorlsFig 19.19b

38. Four Homeotic Floral Genes • AP1 without AG or AP3 induces sepals • AP1 with AP3 induces petals • AP3 with AG induces stamens • AG without AP1 or AP3 induces carpels

39. ABC Model of flower devlopment

40. cont. • Control of Flower formation could evolve with a small number of changes in timing or location of gene expression which could result in large morphological changes in the flower • Coevolution with insects may lead to the rapid selection of some of these changes

41. Clarifying the roles of LFY and its target genes • LFY and its target genes ( AP1, AP3, AG) are not the genes which form the flower but seem instead to indicate the location and timing of floral parts • In fact LFY, AG and MADS-boxes have been identified in non-flowering plants such as pines and ferns • In these other plants the genes involve the formation of reproductive structures but not flowers • Like the HOM/Hoxgenes in animals, the MADS-box genes of plants may have evolved for some other function and then been co-opted later for the control of flower formation

42. DEEP HOMOLOGY • Though Arthropod limbs and tetrapod limbs are not normally considered to be homologous, at a much deeper level in the developmental process controlled by genes, they do share a common control mechanism, the homeotic loci and the developmental genes such as Distal-less.

43. The End

45. Onychophorans Uniramians Chelicerates Trilobites Crustaceans

49. Figure 19.8 a pg 737