The Drive Mechanism Of Aegilops Speltoides B Chromosomes Based On Cytogenetics Analyses

When transmission rates of chromosomes are higher than 0.5,not obeying the Mendelian law of equal segregation,the resulting transmission advantage is collectively referred as "drive".Although supernumerary B chromosomes(Bs)possibly display the most common form of drive found for any genetic element,little is known about the cellular and molecular mechanisms behind their drive.B chromosomes(Bs)are supernumerary chromosomes as dispensable elements in the host which do not recombine with the A chromosomes(As)of the regular complement.In certain species of plants,especially the Gramineae,where it involves directed nondisjunction at pollen mitosis.Here we analyzed the drive mechanism of a wild diploid wheat species Aegilops speltoides Bs during the first mitosis in the male gametophyte and provide direct insight into cytogenetical mechanism(1)To obtain direct insights into the mechanism of B chromosome accumulation we employed a tissue section method for anthers undergoing the first pollen grain mitosis to overcome the problem of the impermeable pollen grain wall that interfered with immunolocalization and in situ hybridization.Centromeric histone H3 variant CENH3 and tubulin antibody were applied to check the reason behind B chromosome nondisjunction during first pollen mitosis.We figured out that nondisjunction of Bs in accompanied by centromere activity(CENH3-positive)and tubulin interacting B sister chromatids occur Unequal cell division might be the reason why Bs accumulate preferentially in the generative nucleus therefore ensures their transmission at a higher than expected rate to next generation.At second pollen mitosis,B sister chromatids disjoin and a normal segregation of Bs occurs(2)To quantify the B accumulation rate in sperm nuclei in a high efficiency way,we combined flow cytometry,immunolabelling and FISH to evaluate the percentage of B-positive generative and vegetative pollen nuclei without B chromosomes.Mature pollens were flow-sorted,derived from plants possessing 1B to 6B chromosomes Independent on the number of Bs present in the mother plants,Bs accumulate in the sperm nucleus with more than 93%,accumulation of Bs happens only in less than 7%of vegetative nuclei.(3)To test whether the(peri)centromere composition differs between A and B chromosomes,eleven(peri)centromeric repeats originated from close related species were determined in Ae.speltoides during mitotic metaphase and meiotic prophase.6C6-3/4,CRW2 and RCS2 possessed a similar abundance in A and B chromosomes,while other like 192 bp,CCS1,Quinta-LTR and Weg 1-LTR are reduced in Bs.In contrast,the TailI repeat possessed an increased copy number in Bs.Two of them pBs301 and centromere satellite repeat(CenSR)were specifically distributed on the pericentromere region of As while totally absent on Bs.It seems that the(peri)centromeric region of both chromosome types possess the same repeats although of different copy numbers which indicated an intraspecific origin of the Bs.The processes of B chromosome drive in Ae.speltoides and rye is similar,suggesting a common origin of the drive process in a shared ancestor of both species.Alternatively,similar accumulation mechanisms evolved independently during the evolution of Poaceae.