Darwin’s contributionsCommon Descent: The branching nature of evolution Natural selection: mechanism which variation among individuals into evolutionarychange. The source of variation is mutationThe phylogenetic tree shows the essential feature of evolution -- that it is branching. (It is exactly like ahuge genealogy. That is, in evolution, individual organisms give rise to progeny, who give rise to further progeny, etc., etc. Thus a branching pattern of ancestor and progeny is produced. There is no linear progression from one species to another. Related groups share common ancestors - the closer thecommon ancestor, the more closely related. No organism living today is the ‘evolutionary ancestor’ of any other living organism.)The importance of understanding developmental biology for thinking about evolution(Continuity of life: descent with modification)evolution over geological time and development of individual Evolution of animals – our place in time and in the diversity of life on earthThe Cambrian radiation – fossils of most of the major animal body plans present today.The Molecules and machinery of lifemeiosis accomplishes:1. Forms haploid gametes from diploid cells 2. Ensures genetic variation of the progenya. Through recombination, scrambles the alleles of genes on the individual chromosomes b. Generates random assortment of each pair of homologous parental chromosomes into the haploid gametesSex Determination Asexual –vs- sexual reproduction Modes of sexual reproductionIn genetic sex determination-specialized sex chromosomes.In environmental sex determination, sex is determined by external signalsSexual reproduction: outcrossing - intermixes two genomes - progeny are genetically different from either parentThe genetic ‘deck’ is shuffled in every generationAsexual reproduction: ‘selfing’ - perpetuates the parental genome - progeny are genetically identical tothe parentChange can happen only through build-up of mutationsDosage compensation systems are present only in sexually reproducing animals that have sex chromosomes. Dosage compensation acts to balance gene expression from the different numbers or kind of sex chromosomes present in males and females. Dosage compensation acts only on sex chromosomes, not on autosomes.Sex determination in vertebratesDiverse upstream signals - but similar downstream eventsVertebrates have an embryo that starts off bi-potential (The gonad is bi-potential gonad, and both male and female presumptive structures are present)Whether sexual development proceeds toward male or female depends on what signals trigger the initiation of sexual developmentThe initial triggers that start sexual development are diverse (in different species, they may be genetic or non-genetic)After the initial triggers, the downstream pathways that carry out sexual development are similar, and all depend on differential gene expression, together with hormone action.In most species, once the initial trigger acts, then sexual development is permanently set towards male or female. However, in some species, bipotential features are retained; in some species of fish, sex maychange in the adult, depending on social cues or other cues from the environment.Sex determination in humans and other mammalsThe mammalian sex determining gene SRY on the Y chromosome triggers male development If different parts of the sex-determining program don’t mesh, then intersex conditions may resultWhy do XX human males and XY human females occur? 1. The X and Y chromosome pair during mitosis and meiosis via a region of very similar DNA calledthe “pseudo-autosomal region.” 2. The human male sex determining gene lies so close to the region where X and Y pair thatabnormal recombination between X and Y chromosome sometimes occurs during meiosis (approximately 1 out of 20,000 XX humans are male). Note that this phenomenon doesn’t occur in mice: the mouse Sry gene is not so close to the pseudo-autosomal region of Y as in humans.3. The hypothesis that XY females and XX males arise by abnormal crossing over was confirmed in the late 1980s by development of molecular probes for Y-specific sequences. Most XX males carry some Y-specific sequences.V. The current view of the human Y chromosome: A. By 1991 we knew that the Y chromosome carries a number of male-specific genes including SRYand other genes expressed in the testis that are necessary for spermatogenesis or other aspects of male development.In 1997 Bruce Lahn and David Page discovered that the human Y chromosome also carries a number of “housekeeping” genes that are expressed in many cell types and needed for general metabolic functions. 1. There are closely related genes to these Y-linked “housekeeping” genes on the X chromosome.The X chromosome copies of these genes escape X-inactivation -- thus both males andfemales have two functional copies of these genes. 2. These observations confirm the evolutionary origin of Y as a “degenerate” X.How does the pathway for mammalian sex determination work? A. The early gonads in embryos of humans and other mammals have both potential male and femalestructures. The gonad in a human embryo is bi-potential up to about 6 weeks of development.B. If SRY is expressed, then genes that regulate male development are turned on and the gonads develop as testis --also, female development is turned off, and presumptive female structures degenerate. Similarly, if SRY is not expressed, then the pathway to female development is turned on; genes that regulate female development are turned on, the gonads develop as ovaries; male development is off; and presumptive male structures degenerate.C. In addition, in mammals, hormonal controls exert substantial influence on sexual development. Thus while the genetic pathway determines the sexual pathway, how development is played out depends to a large extent on action of hormones, in particular testosterone and estrogen, the sex hormones.Different “styles” of sex chromosomes (see examples in the figures for lecture):a. The chromosome names XX and XY are used for species in which the male is the heterogametic sex (has two different sex chromosomes and can give rise to two different kinds of haploid gametes - bearing X or bearing Y).b. For species in which the female is the heterogametic sex, the sex chromosomes are called WZ (the female) and ZZ (male).c. There is not necessarily any relationship between the sex chromosomes in different