University:
Amherst College
Course:
BIOL 220 | Developmental Biology
Academic year:

2020
Views:

425

Pages:

13
Author:

Emilee Weeks

Determination of Germ Cells The Germ Line • Early separation of “germ line” from regular somatic cells in many but not all animals – Early specification of primordial germ cells (PGCs) – Later migration into developing somatic gonad • In gonadal environment, gametogenesis – Meiosis – Sperm/egg differentiation • Germ cell determination can either be – Autonomous – Specified by neighbors Segregation of Germ Plasm of Parascaris • Theodor Boveri (1862-1915) – Nematode of horse and pig intestines – Observed the two chromosomes through cell divisions, through gametogenesis, through development – Chromosome parts loss correlated to special cytoplasm Does a Specific Region of Cytoplasm Protect the Chromosomes from Diminution? • Centrifugation to reorient the first cleavage plane relative to the vegetal cytoplasm centrifuged: Consequences • Only in germ line is all chromosomal information retained • Concept of stem cells – Cells that give rise to one the same and one different Early Segregation of Cytoplasmic Factors Defines PGCs • Segregation of P granules in C. elegans embryo follows PGC lineage • Contain RNA binding proteins and transcription inhibitors germ cell precursor Drosophila Pole Plasm • Pole cells (=PGCs) are the first nuclei to migrate out to periphery and cellularize • Surrounded by pole plasm – mitochondria, fibrils, polar granules, polysomes, etc. – plasm similar in other organisms How do We Know Pole Plasm Specifies the Germline? • Experiments demonstrated importance of pole plasm for generating germline – Hegner (1911) removed posterior pole cytoplasm from beetle eggs -----> beetles grew up to be sterile – Mahowald (1971) prevented pole cell nuclei from reaching posterior pole plasm in Drosophila embryos -----> no germ cells made! – Mahowald (1974) pole plasm transplantation expts • Into anterior makes pole cells there (fate change) How Can We Use Genetics to Identify Components of Pole Plasm? • If mother is unable to pack all components of pole plasm into her oocytes…..? – Her progeny will be sterile – The mom will be “grandchildless” • Hunt for maternal-effect mutations that cause sterility in offspring Components of Pole Plasm • grandchildless mutant identified: – – – – – – – gcl (germ cell-less) look at localization of mRNA and protein Transcribed in nurse cells Transported to oocyte Transported to posterior Translated in early cleavage Goes to nucleus and responsible for germ cell formation Components of Pole Plasm • 1) mRNA of the gcl gene • 2) posterior determinants – also important for ant-post patterning – nanos, oskar, vasa – injection of oskar mRNA into ant of embryo -> ectopic pole cell formation – Excess oskar -> excess no. pole cells – Localizes other proteins and RNAs like nanos – vasa RNA binding protein (helicase) is well conserved and found in germ plasm of many animals Components of Pole Plasm • 3) mitochondrial ribosomal RNA (mtrRNA) – Small and large subunits of ribosome that are specifically associated with mitochondria – Exported to pole plasm and components of the granules – injection of mtrRNA into UV-treated eggs -> restores pole cell formation Components of Pole Plasm • Proteins/RNAs involved in localization and inhibition of transcription/translation • Hypothesis: – Pole cells = cells that are prevented from differentiating – Retain totipotency

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