BSCI222 Lecture 14 10 22 13 missed L 13 Last time chromosome variation Variation in ploidy variation in the number of complete chromosome sets o Haploid 1N diploid 2N etc o Autopolyploids versus allopolyploid o Autopolyploidy happens by non disjunction DNA replicates chromosome separate but the cell never divides Happened in one cell thus auto If 4N happens in a population of individuals just a few of them then their gametes are 2N after meiosis unite with a normal 1N gamete making a triploid offspring Triploids are usually sterile out in the wild used a lot in fish for aquaculture 3 copies of chromosomes makes it very hard to segregate that evenly into gametes For every single chromosome has its own random pattern of segregation making the gametes highly unbalanced Could have 1N of chromosome A 2N of chromosome B 0 for C 0 for D 2N for E etc The organism with such an unbalanced set of chromosomes will not usually survive o Plants have undergone a lot of speciation through both of these mechanisms o Allopolyploids are made by hybridization 2 species gametes fuse Very difficult because of all the differences in chromosome structure for it all to properly pair in meiosis and properly separate Typically make nonviable gametes BUT if nondisjunction happens it leads to a doubling of all chromosomes producing an allotetraploid and then the chromosome pairing and separating into the gametes is balanced During the history of domestication of modern wheat had 2 cycles Einkorn wheat AA 2n 14 and wild grass BB 2n 14 Same number of chromosomes slightly different structures Mitotic nondisjunction balanced gametes The resulting Emmer wheat AABB 4n 28 joined with different wild grass DD 2N 14 making a hybrid ABD 3N 21 which via mitotic nondisjunction made bread wheat AABBDD 6N 42 The people didn t know this was happening was random Chapter 22 Developmental Genetics and Immunogenetics Drosophila life cycle o Drosophila egg is single cell After fertilization have a single diploid nucleus in this cell and a huge mass of cytoplasm o First thing that happens is multiple rounds of nuclear division and DNA replication and chromosomal segregation but no cleavage of the cytoplasm Thus multinucleate cell called a syncytium about 6000 nuclei Molecules can diffuse through this single cell for the early stages of development Have waves of division across the syncytium o After all the nuclear division the nuclei migrate out to the edges of the cell making a skin of nuclei around the cytoplasm The pole nuclei at the end will become the germ line o Later the actual cells form about 100 cells from one end of the embryo to the other o All the molecules in the egg and the DNA sequences encode all the information about what is head what is tail what is up what is down o Right at about 10 hours the number and orientation of the body segments are established Gastrulation Have head thoracic segments and abdominal segments Still a larvae worm o The information and the cells that form the adult structures are segregated in the larvae First have to specify the larvae then later have to specify the adult The early processes that pattern the embryo also pattern the imaginal discs which are segregated off and hold the patterns for the adult body Small discs of tissue which will ultimately grow and become that structure Wing haltere leg etc o Overall 3 main stages 1 Establishment of the main body axes 2 Determination of number and polarity of body segments 3 Establishment of identity of each segment o The egg can t know up from down but the mother left instruction in the egg left an asymmetric distribution of pipe sulfotransferase inside the egg shell on the ventral sides but outside the plasma membrane This works with a number of proteases out there to cleave and produce something called a Spaetzle fragment This fragment binds to the Toll receptor which induces a cascade in the cell which then degrades the cactus protein thereby releasing the dorsal protein throughout the cytoplasm migrating into the nuclei in only the ventral regions which then enters the nuclei and acts as a transcription factor altering the transcription of the nucleus How the mother when making the egg distributes that pipe sulfotransferase is what determines top from bottom Called dorsal because if it s mutated the dorsal protein will work oppositely through the dorsal region o Front to back also instructions from the mom In the ovary when the egg is being formed bicoid mRNA is being deposited in the anterior end of the embryo anchored by proteins and not translated Once fertilization happens that mRNA is released and starts being translated which makes bicoid protein The bicoid protein then diffuses throughout the embryo not restricted the way the protein was diffused throughout entire syncytium but concentrated at the anterior end At the posterior end of the egg mom left nanos mRNA same way when translated the nanos protein becomes concentrated at the posterior end of the egg and inhibits formation of anterior structures Both proteins have gradients across the embryo in opposite directions Essentially have 3 regions high in bicoid high in nanos and low in both The hunchback gene which is upregulated by bicoid and downregulated by nanos has expression high when there is bicoid and low when it encounters nanos Individual cells across this axis with their concentration factors of both of the proteins causes a hunchback gradient which makes more regions high B high H high H low B low H low N low H high N The promoters of genes are like a digital calculator have a series of binding sites that bind these some acting as transcriptional activators and some as repressors Within some cell in high region of bicoid and hunchback are activators tendency to turn on this gene in this region If in a region of high bicoid and high hunchback but also high Giant repressor gene is not expressed here Kruppel is established via nanos much like hunchback is from bicoid Kruppel is also a repressor Getting a digital readout of the program in the enhancer Even skipped eve expression is limited by Giant and Kruppel turning it off on either side resulting in a very narrow band of even skipped expression in the middle of the embryo called the even skipped stripe 2 present in 7 stripes across the embryo but we were just looking at one stripe and enhancer This enhancer corresponds to a small segment of the eve gene This enhancer is doing the computation of the concentration of