Plant Tax II Week 2

Key concepts of Week 2

What are prasinophytes? What are the differences among Ulvophyceae, Trebouxiophyceae and Chlorophyceae?

Week 2 topic

Chlorophytes: Prasinophytes, Ulvophyceae, Trebouxiophyceae and Chlorophyceae

Phylum/division name of 'chlorophytes'

Chlorophyta (Infrakingdom Chlorophyta in Viridiplantae)

Which green algae were excluded from Chlorophyta, and why?

Closterium, Chara, Coleochaete etc. - more closely related to land plants - transferred to Streptophyta

Previously, how were green algae classified?

All green algae were classified in the Chlorophyta

Characteristics of Chlorophyta (flagella)

Flagellate cells usually have apical and opposite flagella and an eyespot*

Characteristics of Chlorophyta (flagellar apparatus)

Radially symmetrical cruciate flagellar apparatus*

Characteristics of Chlorophyta (mitosis/cytokinesis)

Closed mitosis, usually cytokinesis with phycoplast*

Characteristics of Chlorophyta (photorespiration)

Glycolate dehydrogenase in mitochondria

Characteristics of Chlorophyta (cellulose synthase)

Linear cellulose synthetase complex

Which Chlorophyta features have exceptions (*)? Where are exceptions found?

Flagella/eyespot, flagellar apparatus, mitosis/cytokinesis (phycoplast) | exceptions are especially in prasinophytes

Chlorophyta is composed of...

UTC clade (core chlorophytes) + some early divergent lineages (prasinophytes)

UTC clade includes...

All chlorophytes excluding prasinophytes

Etymology of 'prasinophytes'

'prasinous' = leek green

What are prasinophytes?

Early divergences (early divergent lineages) in green plants / Chlorophyta | considered 'primitive' green plants

Prasinophytes originally included...

Green flagellates covered by organic scales

Which species were subsequently added to prasinophytes?

Naked species and non-motile walled species

Number of prasinophyte species

ca. 170 species

Prasinophyte flagellate cell structures (typical, e.g. Tetraselmis-type)

Flagellar scales, flagella with blunt ends, flagellar pit, body scales

Prasinophyte genera shown in lecture

Micromonas, Mamiella, Pyramimonas, Nephroselmis, Prasinococcus, Mesostigma, Bathycoccus, Tetraselmis, Pterosperma

Prasinophytes are monophyletic or not?

Paraphyletic (early divergences of Chlorophyta, plus Mesostigma at the base of Streptophyta)

Which prasinophyte is at the base of Streptophyta?

Mesostigma (Mesostigmatophyceae) - discussed in week 3

Old class name for prasinophytes and why it was abandoned

Prasinophyceae (or Micromonadophyceae) | not monophyletic, now split into many classes

Prasinophyte classes/lineages (from slides)

Mesostigmatophyceae, Scourfieldiales / 'Pseudoscourfieldiales', Mamiellophyceae (Palmophyllophyceae), Pyramimonadales, Nephroselmidophyceae, Chloropicophyceae, Picocystophyceae, Pedinophyceae, Chlorodendrophyceae

Why are prasinophytes considered primitive? (1st reason)

They are very diverse in many features (flagellate cell structure, flagellar apparatus, mitosis, cytokinesis, pigments, physiology), which are nearly constant in other green plant groups

Why are prasinophytes considered primitive? (2nd reason)

Some prasinophytes show an intermediate state between typical chlorophytes and streptophytes

Interpretation of prasinophyte diversity

They retain the early diversity in the evolution of green plants

Flagella in core chlorophytes (insertion, direction, number, motion)

Apical, opposed, 2 or 4 (or many), homodynamic

Flagella in core streptophytes (insertion, direction, number, motion)

Subapical, parallel, 2 (or many), homodynamic

Flagella in prasinophytes

Lateral or apical | opposed or parallel | 1, 2, 4, 8 or 16 | heterodynamic or homodynamic (very diverse | some intermediate)

Ancestral flagellar condition of green plants

Unicellular flagellate with heterodynamic two flagella (anterior new/immature no.2 and posterior old/mature no.1), inserted laterally

Why is the heterodynamic two-flagella state thought ancestral?

Eukaryotic flagella are distributed semiconservatively: each daughter cell receives one mother flagellum (posterior, no.1) and makes one new flagellum (anterior, no.2) | this configuration is nearly universal in eukaryotes (stramenopiles, glaucophytes, Amoebozoa, Alveolata, Excavata, Opisthokonta)

Semiconservative flagella distribution

On division, each daughter cell receives half of the mother flagella/basal bodies (old = posterior no.1) and the other half arises de novo in cell division (new = anterior no.2)

Flagellar apparatus in prasinophytes

Very diverse: some cruciate symmetrical like core chlorophytes, others asymmetrical like core streptophytes | some have MLS (multilayered structure) like streptophytes

Phagocytosis in green plants

Most green plants cannot phagocytose (they absorb organic matter) | only some prasinophytes perform phagocytosis - a relic of the ancestral eukaryotic feature

Cell covering of prasinophytes

Usually covered by organic scales formed in the Golgi body | some naked or covered by cell wall

Importance of prasinophyte organic scales

Scale morphology/construction is very diverse and an important taxonomic feature

Evidence that organic scales are ancestral in green plants

Some core chlorophytes and streptophytes also have a certain type of prasinophyte scale

Prasinophyte habitat

Mostly marine, some freshwater (e.g. Matsumiike pond)

Phycomata

Prasinophyte cysts | very rich microfossils (e.g. Devonian) in the Paleozoic

Role of prasinophytes in geological history

Probably major marine phytoplankton before the Mesozoic era (rich Paleozoic fossil record)

Major marine phytoplankton today

Secondary plants such as diatoms, dinoflagellates and haptophytes (not green algae)

When did the marine producers change from primary to secondary plants?

Drastic change in the Mesozoic era

What does UTC stand for?

Ulvophyceae + Trebouxiophyceae + Chlorophyceae (UTC clade = core chlorophytes)

UTC clade: size

The largest and most diverse clade in the Chlorophyta

UTC flagellate cell shape and flagella

Basically tear-drop shaped, nearly radially symmetrical | two (or four) flagella emerging oppositely from the anterior end | eyespot at fixed side (not strictly symmetrical)

Unique axoneme feature of UTC clade flagella

Absence of one dynein arm (no.1 outer dynein arm) in the axoneme

Possible function of missing dynein arm in UTC

Probably related to flagellar beating in a single plane

Flagellar apparatus of UTC clade

Radially symmetrical cruciate type, with variation in arrangement of basal bodies

CCW type flagellar apparatus

Basal bodies arranged counterclockwise (11 and 5 o'clock type)

DO type flagellar apparatus

Basal bodies directly opposed (0 and 6 o'clock type)

CW type flagellar apparatus

Basal bodies arranged clockwise (1 and 7 o'clock type)

Which basal body arrangement is considered ancestral in UTC? Why?

CCW | because prasinophytes also have a CCW flagellar apparatus

UTC clade: mitosis

Closed mitosis

UTC clade: persistent spindle vs collapsed spindle

Persistent interzonal spindle → cytokinesis without phycoplast | collapsed interzonal spindle (daughter nuclei get closer) → cytokinesis with phycoplast

Phycoplast

Microtubules parallel to the division plane (formed after collapse of the interzonal spindle)

UTC comparison: Organization (U / T / C)

Unicellular to multicellular (all three)

UTC comparison: Flagellar apparatus (Ulvo / Trebo / Chloro)

CCW / CCW / DO or CW

UTC comparison: Telophase spindle (Ulvo / Trebo / Chloro)

Persistent / Collapsed (non-persistent) / Collapsed (non-persistent)

UTC comparison: Phycoplast (Ulvo / Trebo / Chloro)

Absent / Present / Present

Which UTC class has a persistent telophase spindle and no phycoplast?

Ulvophyceae (also found in most prasinophytes and streptophytes)

Which UTC class has DO or CW flagellar apparatus?

Chlorophyceae

Which two UTC classes share a CCW apparatus but differ in spindle/phycoplast?

Ulvophyceae and Trebouxiophyceae (Ulvo: persistent spindle, no phycoplast | Trebo: non-persistent spindle, phycoplast)

Which two UTC classes share non-persistent spindle + phycoplast but differ in flagellar apparatus?

Trebouxiophyceae (CCW) and Chlorophyceae (DO or CW)

Ulvophyceae: defining characteristics

CCW flagellar apparatus | closed mitosis with persistent telophase spindle | cytokinesis by furrowing with NO phycoplast

Ulvophyceae: ancestral feature question

Persistent spindle/absence of phycoplast: ancestral feature or secondary reversal? (as other UTC have collapsed spindle and phycoplast)

Ulvophyceae: number of species

ca. 1,700 species

Ulvophyceae: organization

Mostly macroscopic green seaweeds (multicellular or siphonous) | some microscopic unicellular, colonial or filamentous algae

Ulvophyceae morphogenesis (Ulva, Gayralia)

Axenic (bacteria-free) culture cannot form a normal body | with bacteria they can | the bacterial substance is 'thallusin' (thallus = body of algae) | removal of thallusin causes decomposition

Thallusin

Substance produced by bacteria required for normal morphogenesis of some seaweeds (e.g. Ulva)

Siphonous body

Body without septa - a giant single (multinucleate) cell | found in Bryopsidales (Ulvophyceae)

Caulerpa body

Root-, stem- and leaf-like 'organs' but no septa (continuous internally) | one species is 'sea grape' (Umibudo), a local food in Okinawa

Codium body

Body of intertwined filaments without septa | 'tissue' differentiation: central medulla without plastids and surface utricles (swollen structures) with many plastids

Utricle / medulla (Codium)

Utricle: swollen surface structure containing many plastids | medulla: central part containing no plastid

Ulvophyceae life cycle

Very diverse | mostly haplodiplontic (G = S, G > S or G < S) | gametophyte produces biflagellate gametes (2 flagella) | sporophyte produces quadriflagellate zoospores (4 flagella)

Isomorphic vs heteromorphic in Ulvophyceae

Isomorphic: gametophyte and sporophyte same size/appearance | heteromorphic: gametophyte larger or smaller than sporophyte

Ulvophyceae habitat

Mostly marine | some freshwater (Marimo / Cladophora ball, Lake Akan), some terrestrial, some epizoic/symbiotic

Trentepohliales

Exclusively terrestrial ulvophytes | accumulate carotenoids (orange), osmoregulation materials, and have plasmodesmata - adaptations to terrestrial habitat | e.g. Trentepohlia

Trichophilus

Epizoic ulvophyte living on hairs of three-toed sloths (Bradypus), making them green | hairs accumulate water | benefit for sloth uncertain (camouflage? food?) | coevolved with sloths | forms microecosystem with moths (Cryptoses), fungi and bacteria

Ulvophyceae used as food

Monostroma (tsukudani of nori), Ulva (aonori, green powder on yakisoba/okonomiyaki), Capsosiphon, Caulerpa (sea grape, cultivated in greenhouses)

Green tide

Mass growth of Ulva in eutrophic coastal waters | detrimental to coastal ecosystems and human activities | removal also removes excess nutrients | possible biomass resource

Killer algae

A mutant of Caulerpa taxifolia (Bryopsidales)

Killer algae: traits

Tolerates low temperature and light, attaches to various substrates, large, active vegetative reproduction, more toxic to herbivores (caulerpenyne)

Killer algae: history

Accidentally released into the Mediterranean Sea in 1984 | also invaded California, Australia, Japan | dense mats (130 km2) prevent native species growth | decreased in the Mediterranean since 2000 while other invasive Caulerpa increased

Ulvophyceae genera shown

Ulva, Gayralia, Codium, Caulerpa, Monostroma, Capsosiphon, Cladophora (Marimo), Trentepohlia, Trichophilus

Trebouxiophyceae: defining characteristics

Biflagellate cells with CCW flagellar apparatus | closed mitosis with NON-persistent telophase spindle | metacentric spindle | cytokinesis by centripetal furrowing with phycoplast

Metacentric spindle

Spindle in which centrioles are situated not at the poles but laterally, near the division plane (Trebouxiophyceae)

Trebouxiophyceae: how described / when

Described relatively recently (1995) based on ultrastructural and molecular data

Trebouxiophyceae: species number

ca. 700 species

Trebouxiophyceae: organization

Non-motile vegetative stage: unicellular, colonial, filamentous, blade-like or siphonous | mostly microalgae

Trebouxiophyceae genera shown

Trebouxia, Chlorella, Apatococcus, Microthamnion, Planctonema, Botryococcus, Lagerheimia, Actinastrum, Phyllosiphon, Prasiola, Coccomyxa

Trebouxiophyceae habitat

Mostly freshwater or terrestrial (very common in terrestrial habitats, e.g. green mats on walls and tree bark) | some marine | also in nuclear reactors and inside other organisms

Trebouxiophyceae symbiosis

Many are symbionts: amoebae, labyrinthulids, cercozoans, heliozoans, ciliates, sponges, hydra, sea anemones, insects, bivalves, seed plants (Araceae, Ginkgo), fungi (lichens)

Well-studied ciliate hosting trebouxiophyte endosymbionts

Paramecium bursaria

Origin of the name Trebouxiophyceae

Trebouxia - the most common lichen photobiont genus

Lichen definition

Fungi (mycobionts) with mutualistic symbiotic algae (photobionts)

Lichen: type of symbiosis

Extracellular symbiosis - photobionts are not intracellular but attached to hyphae (fungal filaments)

Lichen reproduction

By symbiotic units (soredia) | fungal spores must get a photobiont soon after germination (re-lichenized)

Soredium

Symbiotic unit (fungal hyphae + algae) used for lichen vegetative reproduction

Lichen growth forms

Foliose (leafy), fruticose (shrubby), crustose (crust-like)