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The Science of Plant Systematics

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Title: The Science of Plant Systematics


1
The Science of Plant Systematics
  • Plant Systematics (PBIO 309/509)
  • Harvey Ballard

2
Traditional Meaning of Plant
  • Autotrophs by photosynthesis
  • Chlorophyll A, B
  • Storage of carbohydrates (mostly starch)
  • Includes green algae (Chlorophyta) and land
    plants
  • Previously included fungi and related groups,
    these now removed as lineages nearer to animals

3
Plant Groups Covered in This Course
  • Extant land plants
  • liverworts
  • hornworts
  • mosses
  • vascular plants (tracheophytes)
  • Course touches briefly on ferns allies and
    gymnosperms
  • Focuses on
  • angiosperms

Judd et al. (2002)
4
What is Systematics?
  • Aims to recognize, describe, name, distinguish,
    relate and classify earths organisms
  • Borrows from other fields--very much a
    multidisciplinary, or hybrid, discipline
  • Supplies evidence to evolutionary biology,
    ecology and other fields
  • Fundamental to all other scientific endeavors
    (and many non-scientific human concerns)

5
Why is Systematics Fundamental?
  • Why do we give names to entities?
  • Who cares if different labs studying mutations in
    Arabidopsis thaliana, or investigating genetic
    disease in chimpanzees, work with the same
    organism across labs? How do we know?
  • How do we access information in libraries and
    museums, in computer or cabinet files, or on the
    internet?

6
What is Systematics?
  • Uses diverse approaches
  • Morphology
  • Anatomy
  • Palynology
  • Microscopy
  • Biochemistry
  • Molecular Biology
  • Genetics
  • Physiology
  • Ecology
  • Evolution
  • Bioinformatics

7
Why is Systematics Important?
  • Detailed information at all hierarchical levels
    is key to most scientific fields, medicine and
    numerous aspects of human society
  • Names of taxa (e.g., species), or even
    individuals, are tags for information retrieval
    and knowledge synthesis

8
Why is Systematics Important?
  • Modern systematic studies provide biological
    context to evolutionary and ecological studies
  • Modern classifications are predictive, can guide
    bioprospecting for medicines, foods, etc.
  • Species-level information can guide conservation

9
The Practice of Systematics
  • Systematics sensu stricto
  • Determination of distinct taxa using diverse
    evidence
  • Inference of relationships using phenotypic or
    genetic data
  • Classification of taxa into larger groups
  • Production of systematic revisions, phylogenies,
    classification systems

10
The Practice of Systematics
  • Systematics sensu stricto
  • Name increasingly restricted to molecular
    systematics (more sexy, generally more fundable
    than unadulterated traditional studies), commonly
    focused at or above family level
  • Species-level systematics uncommon
  • Extrasevolutionary or biogeographic hypotheses
    can be addressed empirically
  • Common at larger universities, largest museums
    (few doing it)

11
The Practice of Systematics
  • Taxonomy
  • Nomenclatureapplication of names (follows
    international rules)
  • Characterization and distinction of taxa from
    field and herbarium studies
  • Production of monographs, floristic treatments,
    checklists
  • Common in herbaria and museums, small universities

12
The Practice of Systematics
  • Above two subdisciplines fall along a continuum
  • Many botanists fall into one or other category
  • Determined partly by resources of individuals and
    institutionstraining, institutional aims, time,
    money
  • Collaboration spans chasms between molecular
    systematists who are not experts in a group and
    experts lacking resources to do molecular
    systematics

13
Phylogenetic Approach in This Course
  • Course uses current APG (Angiosperm Phylogeny
    Group) classification as framework to survey
    angiosperm families
  • Based heavily on Judd, et al.s Plant
    systematicsA phylogenetic approach, 2nd ed.
    (2002), supplemented by Angiosperm Phylogeny
    Website, etc.
  • Facilitates understanding of evolutionary change
    going up the tree
  • Covers families in southeastern Ohio

14
The Phylogenetic Approach
  • Phylogeny--branching tree revealing
    relationships of taxa (species, genera, etc.)
  • Known taxa at branch tips, connected by
    hypothetical ancestors
  • Generated from diversity of data, commonly DNA
    sequences
  • More on algorithms later

Judd et al. (2002)
15
The Phylogenetic Approach
  • Three types of relationship possible
  • Monophyleticcommon ancestor all descendants
    (natural)
  • Paraphyleticcommon ancestor some descendants
    (artificial, generally rejected)
  • Polyphyletic--some descendants ancestor
    (artificial, rejected)
  • Monophyletic groups the only natural taxa
  • Para- and polyphyletic groups demand shifting
    taxa around, or merging groups to achieve
    acceptable classification

16
The Phylogenetic Approach
A monophyletic
B paraphyletic
AB polyphyletic
Judd et al. (2002)
17
The Phylogenetic Approach
  • Genetic (DNA-based) data ideally used for
    phylogeny reconstruction where available
  • Molecular data (in form of As, Cs, Gs and Ts)
    provide numerous characters for evaluation of
    relationships
  • Molecular phylogeny provides non-circular basis
    for reexamining other evidence (e.g., phenotypic
    traits)
  • More on this later

18
The Phylogenetic Approach in Practice
  • Monophyletic groups retained
  • Others recircumscribed
  • Alternative endpoints along continuum
  • Lump all taxa in broader group
  • Subdivide more finely

Judd et al. (2002)
19
The Phylogenetic Approach in Practice
  • Example 1
  • Monocots monophyletic
  • Monocots nested within dicots
  • Dicots paraphyletic with respect to monocots

Judd et al. (2002)
20
The Phylogenetic Approach in Practice
Basal Dicots
  • Solution to Example 1
  • Retain Monocots
  • Recognize Basal Monocot lineages
  • Recognize Eudicots

Magnoliids
Monocots
Eudicots
Judd et al. (2002)
21
The Phylogenetic Approach in Practice
Basal Dicots
  • Solution to Example 1
  • Higher-level groupings also supported by
  • Embryology
  • Major biochemical compounds
  • Pollen types

Magnoliids
Monocots
Eudicots
Judd et al. (2002)
22
The Phylogenetic Approach in Practice
  • Example 2
  • Genus Hybanthus is 3rd largest in the
    Violaceaeup to 125 spp.
  • Similar in gross floral features, herb to shrub
    habit

H. monopetalus (Gordon, photo)
H. communis
H. concolor (Barnes, photo)
23
The Phylogenetic Approach in Practice
  • 92-112 species worldwide
  • Diversity hotspots in N. Mexico, West Indies,
    S.E. Brazil/Paraguay, E. Africa and S. Australia

24
The Phylogenetic Approach in Practice
  • Hybanthus is highly polyphyletic
  • Merger across family would lump extensive
    phenotypic diversity
  • Investigation of Hybanthus initiated

25
The Phylogenetic Approach in Practice
  • Hybanthus groups differ dramatically in
  • Flower symmetry
  • Stamen morphology
  • Seed morphology
  • Chromosome number
  • Pollen morphology
  • Xylem morphology
  • Similar only in
  • expanded bottom
  • petal

26
The Phylogenetic Approach in Practice
Trait Corolla zygomorphy (lateralbottom petal
length ratio) Pombalia 0.33-0.71
0.8-1.00 Hybanthus 0.90-1.00 H. fruticulosus
complex 0.89-0.95 H. thiemei complex 0.50-0.55 H.
enneaspermus comp. 0.38-0.66 Cubelium 0.75-0.80
Pigea 0.30-0.66
27
The Phylogenetic Approach in Practice
  • Trait Attachment of staminal glands on filament

medial attachment
basal attachment
Red line is Base of filament
H. fruticulosus H. enneaspermus Pigea
complex complex
28
The Phylogenetic Approach in Practice
  • Trait Seeds, in relative size proportion

Pombalia Hybanthus
H. fruticulosus complex
H. thiemei H. enneaspermus
Pigea Cubelium complex complex
29
The Phylogenetic Approach in Practice
Summary of 12 Traits at a Glance
Pombalia
Hybanthus 0
H. fruticulosus comp.
H. thiemei comp.
H. enneasp. comp.
Cubelium 0
Pigea
30
The Phylogenetic Approach in Practice
  • Cryptic genera lumped earlier based on gross
    flower similarities
  • Clades are distinct biogeographic units
  • Hybanthus 4 New World genera, 3 Old World
    ones
  • Each molecular clade distinct genus
  • 4 have earlier names, 3 require new ones

31
References
  • Judd, W. S., C. S. Campbell, E. A. Kellogg, P. F.
    Stevens, and M. J. Donoghue. 2002. Plant
    systematicsA phylogenetic approach, 2nd ed.
    Sinauer Associates, Sunderland, MA. pp. 1-11.
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