Transcript Protists
Last day… - introduced the diversity and characteristics of ‘prokaryotes’ - not a monophyletic group, actually 2 whole domains (Bacteria & Archaea); includes anything that lacks more complex cell structure of eukaryotes - today, briefly talk about significance of ‘prokaryotes’… Prokaryotes play essential roles for all living things, including us... - e.g. decomposition and chemical recycling e.g. nitrogen fixation... Prokaryotes form symbioses with many other species Many bacteria are important human pathogens (about 50% of human diseases) But bacteria also have diverse uses in industry, medicine food production, etc. And thermophilic bacteria make much of forensic science & molecular research possible through polymerase chain reaction (PCR) The diversity of protists Formerly considered a Kingdom, but would be paraphyletic - how many kingdoms? Protists exhibit more structural & functional diversity than any other group of eukaryotes - most unicellular, but some colonial or multicellular At the cellular level, many protists are very complex (most elaborate of all cells?) - must carry out same basic functions as multicellular organism, with organelles instead of organs Diverse means of obtaining nutrition… Photoautotrophs have chloroplasts for photosynthesis Ulva Heterotrophs may absorb organic molecules or ingest food particles Amoeba Mixotrophs combine photosynthesis & ingestion Euglena Protists that photosynthesize often called algae, those that ingest food often called protozoa (and absorptive protists have been mistaken for fungi), but these are not distinct taxonomic groups Protists occupy many habitats (usually with water), have varied life cycles & reproduction, etc. golden algae ciliates plasmodial slime mold As eukaryotes, protists have much more complex cells than prokaryotes – how did this happen? Answer may be house guests... May have been ingested as food, or may have been parasite, but somehow smaller aerobic bacteria got inside larger cell - evolved into mitochondrion - if larger cell was anaerobe in environment with O2 increasing may have been beneficial for both Plant cells are still more complex - have chloroplasts and other plastids Serial Endosymbiosis Theory proposes that that chloroplasts arose from ingestion of a photosynthetic bacteria - also may be beneficial for both partners Lynn Margulis proposed theory in 1967 Support from structure of organelles: - size similar to bacteria - own DNA in circular loop, without associated proteins - ribosomes similar to bacterial - fission-like reproduction Additional support from plausibility: many important symbioses, e.g. lichens, coral & algae What bacteria involved? - comparison of DNA sequences show that chloroplasts are derived from cyanobacteria & mitochondria are alpha proteobacteria Mitochondria & plastids seem somewhat independent (own DNA, protein-making machinery, reproduction) but really integrated as part of cell - many of their genes transferred to nucleus In some eukaryote lineages, appears to be secondary endosymbiosis, with a eukaryote (red or green algae) being taken in by a heterotrophic eukaryote These plastids may have additional membranes & a vestigial nucleus Also, proposals that other features of eukaryotic cells result from symbioses (e.g. flagella), but these ideas have less support Survey a few important ‘protists’… First big group, the Excavata, includes 4 important taxa Diplomonads & parabasalids lack plastids, have reduced mitochondria (often anaerobic), lacking ‘key’ enzymes Diplomonads have 2 nuclei & multiple flagella - Giardia causes ‘beaver fever’ Parabasalids include Trichomonas, common STD - moves with undulating membrane & flagella - may have picked up genes from bacteria Euglenozoans include euglenids & kinetoplastids - have spiral or crystalline rod inside flagella Euglenids often autotrophic but can absorb nutrients, some ingest prey - eyespot and light detector to move toward light Kinetoplastids free-living or parasites - Trypanosoma causes sleeping sickness, 1/3 of genome for making different surface proteins - a different Trypanosoma causes Chagas’ disease in Latin America Alveolates make up a diverse group with sacs ( of unknown function) just below plasma membrane - part of larger group known as Chromalveolata - Alveolates include dinoflagellates, apicomplexans & ciliates Dinoflagellates have ‘armor’ of cellulose & 2 flagella in grooves (spin as they swim) - abundant phytoplankton (marine & freshwater), but many mixotrophic or heterotrophic - some may cause ‘red tides’ - some spp. symbiotic with coral, known as zooxanthellae Apicomplexans include Plasmodium, cause of malaria - very complex life cycle, sporozoites injected by mosquito invade liver cells - merozoites released from liver cells, invade red blood cells - burst out after 48 or 72 hours - some merozoites form gametocytes, which produce gametes for sexual reproduction - fertilization in mosquito gut, zygote forms oocyst - sporozoites migrate to salivary glands Ciliates have short cilia in rows, tufts or over whole body - Paramecium uses cilia to sweep bacteria into oral groove - note presence of micronucleus & macronucleus (many copies of genes, transcribed for ‘day to day’ use) Reproduce by binary fission, but undergo conjugation to exchange micronuclei - also gets rid of ‘useless’ DNA Stramenopiles include several groups which have a hairy flagellum & (usually) a smooth flagellum - flagellae usually just on reproductive cells - Oomycetes, diatoms, golden algae & brown algae Oomyctes (water molds, white rusts, downy mildews) are decomposers or parasites - resemble fungi in having multinucleate filaments ` (convergent) - Phytophthora causes potato late blight Diatoms - unicellular ‘algae’ have glass-like shells with overlapping lids - very strong - abundant and diverse (100,000 species?) in salt & fresh water - mostly asexual reproduction Diatomaceous earth (fossil deposits) used for filtering, abrasives, in nail polish, as pesticide, etc. Golden algae - named for yellow & brown pigments - usually unicellular, but may be colonial - photosynthetic or ‘mixotrophic’ Brown algae (Phaeophyta) are multi-cellular & often large - these ‘seaweeds’ have a thallus (body) with parts analagous to land plants: blade – like leaf stipe – like stem holdfast – like root …but simpler structure Postelsia – ‘sea palm’ - common in temperate marine waters Species of kelp up to 60 m long, form ‘forests’ in subtidal areas Some brown algae show alternation of generations - large plant produces zoospores (asexual) - develop into small male or female gametophytes (sexual) - zygote grows into large sporophyte generation