Development and Evolution“A great deal of the problem of neo-Darwinian theory is that it is strictly a theory of genes, yet the phenomenon that has to be explained in evolution is that of the transmutation of form. True, genes may be invented to account for the selection of any desired form, but the real solution to the problem lies in that uncharted realm between genes and morphology.” M. W. Ho and P. T. Saunders. 1979. Beyond neo-Darwinism — an epigenetic approach to evolution. J. Theoretical Biology 78:573-591.The morphologists’ complaintMorphology is epigenetic“Evo-Devo”Homeotic genes and pattern formationHox genes in animalsHox genes in DrosophilaHox genes in Drosophila (Gerhart and Kirschner 1997) (Fig. 18.1)Hox gene mutant phenotypesThe phylogenetic position of Hox genesHox genes in various animal phyla (Fig. 18.3)Changes in Hox expression: arthropod segmentationHox expression and arthropod segmentation (Knoll and Carroll 1999) (Fig. 18.5)The origin of the tetrapod limbLobe-finned fish and the tetrapod limb (Figs. 18.6 and 18.7)The developing tetrapod limb bud AER = apical ectodermal ridge (Fig. 18.8)The development of the tetrapod limb -1The development of the tetrapod limb -2The development of the tetrapod limb -3Arthropod limbs (Brusca and Brusca 2002) (Fig. 18.12)Genetic control of limb formation in arthropodsDeep HomologyMADS-box homeotic genes and development of flowersParts of a flower (Fig. 18.15)The ABCs of flower development mutations (Coen 1999) (Fig. 18.16)A conceptual model of flower formation by homeotic genes (Parcy et al. 1998) (Fig. 18.18)Genes and development – summary1Development and EvolutionChapter 182“A great deal of the problem of neo-Darwinian theory is that it is strictly a theory of genes, yet the phenomenon that has to be explained in evolution is that of the transmutation of form. True, genes may be invented to account for the selection of any desired form, but the real solution to the problem lies in that uncharted realm between genes and morphology.”M. W. Ho and P. T. Saunders. 1979. Beyond neo-Darwinism — an epigenetic approach to evolution. J. Theoretical Biology 78:573-591.3The morphologists’ complaint•The modern synthesis, which has dominated much evolutionary thinking since the mid-20th Century, is a synthesis of Darwinian verbal argument and mathematic population genetics – it seeks to explain evolutionary change ultimately in terms of forces acting to change allele and genotype frequencies in populations•Population genetics thinking does not, and cannot, explain much of what is interesting about evolution – particularly the evolution of morphology of multicellular animals4Morphology is epigenetic•Morphology results from interaction between many gene products and between gene products and the environment and is expressed only through development ( = ontogeny)•We can’t understand the evolution of morphology simply by reference to forces that change allele and genotype frequencies in populations, or simply by understanding how a sequence of DNA nucleotides specifies a sequence of amino acids5“Evo-Devo”•Animal body plans•Formation of limbs in vertebrates and arthropods•Evolution of the flower6Homeotic genes and pattern formation•Homeotic loci are genes that are responsible for telling cells where they are spatially in a developing 4 -dimensional embryo, for telling cells where they are in a developmental sequence, and for determining the fates of cells•In animals, the key homeotic loci are called Hox (for “homeobox”) or HOM genes – they are a gene family created by gene duplication events•In plants, the key homeotic genes are the MADS-box genes•Although there are Hox homologues in plants and MADS-box homologues in animals, Hox loci and MADS-box loci are not homologous to each other7Hox genes in animals•Found in all major animal phyla•Occur in groups (gene duplication events) – the number of genes in each group and the total number of groups varies among phyla•Perfect correlation between the 3’ – 5’ order of genes along the chromosome and the anterior to posterior location of gene products in the embryo. Genes at the 3’ end are also expressed earlier in development and in higher quantity than genes at the 5’ end – spatial, temporal, and quantitative colinearity•Each locus within the complex contains a highly conserved 180 bp sequence, the homeobox, that codes for a DNA binding motif – Hox gene products are regulatory proteins that bind to DNA and control the transcription of other genes8Hox genes in Drosophila•Two clusters – Antennapedia and bithorax•Mutations in the Antennapedia genes affect the anterior of the developing embryo, mutations in bithorax genes affect the posterior•Flies missing one or more Hox gene products produce segment-specific appendages such as legs or antennae in the wrong place•Gene products from Hox loci demarcate relative positions in the embryo, rather than coding for specific structures – for example, they specify “this is thoracic segment 2” rather than “make wing”9Hox genes in Drosophila(Gerhart and Kirschner 1997) (Fig. 18.1)10Hox gene mutant phenotypes•Top: normal fly on left; antennapedia mutant phenotype on right•Bottom: bithorax mutant phenotype11The phylogenetic position of Hox genes•Although Hox genes are expressed in a segment-specific way in arthropods, they are also found in non-segmented animals – they are not “segmentation genes”•Hox genes specify anterior – posterior and dorso – ventral axes in bilateral animals, but homologues are present in sponges and jellyfish, and plants and fungi•The original gene duplication event that produced the Hox complex may have preceded the evolution of multicellularity in animals•10 Hox loci probably existed in the common ancestor of all bilaterally symmetric animals – sponges and cnidarians have just 3 – 4 Hox loci•There is a rough correlation between the number of homeotic loci and complexity of metazoan body plans• Vertebrates have 4 Hox clusters, but other deuterostomes have just a single cluster12Hox genes in various animal phyla (Fig. 18.3)13Changes in Hox expression: arthropod segmentation•Does variation in Hox gene expression correlate with morphological diversity in arthropods?•All arthropods (+ onychophorans) have the same 9 Hox genes•Addition of sequences coding for an alanine region in the product of Ubx may be