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Angiosperm phlogeny group taxonomy classification

The document summarizes the recent APG system of plant classification based on phylogenetic analysis. It describes the following key points: 1. The APG system adopted in 1998 uses DNA sequences and morphology to arrange angiosperms into monophyletic groups or clades at different hierarchical levels, from informal groups to orders and families. 2. Major clades include basal angiosperms, magnoliids, eudicots, rosids, and asterids. Monocots are placed within basal angiosperms. 3. The classification recognizes relationships between groups more accurately than previous systems, adopting the phylogenetic principle of monophyly for formal taxonomic ranks. It is based on evidence from multiple sources

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RECENT SYSTEM OF CLASSIFICATION
Hierarchy of Classification
A BRIEF INTRODUCTION Earlier plant classification ----- Artificial system. From 1990’s started using --- Phylogentic analysis to classify the plants. Phylogenetic character used for Classification are: Rapidly accumulating DNA sequences from the plastid gene rbc L (this gene codes for large subunit of RUBISCO ENZYME).  cp DNA mat K gene ( maturase enzyme encoding gene located with in the intron of chloroplast trn K gene).  Mitochondrial gene atp A (atp genes providing information for making transporter proteins called ATPases) and 18 Sr DNA (satellite related DNA) gave informative data useful in plant classification Further development of polymerase chain reaction techniques which help in bio-chemical as well as anatomical analysis has paved a way for a new system of plant classification.
DNA barcoding is a taxonomic method that uses a short  genetic marker in an organism's DNA to identify it as  belonging to a particular  species . Applications include, for example, identifying plant leaves even when flowers or fruit are not available DNA barcodes that would be linked to the named specimens. This provides a new master key for identifying species, one whose power will rise with increased taxon coverage and with faster, cheaper sequencing".
Preface of the APG System of classification According to traditional system of classification Angiosperms are divided into Monocots and Dicots based on presence of one or two cotyledons. But recent study based on Phylogentic analysis ie., nuclear, plastid and mitrochondrial DNA sequences and morphology do not support this dicotomy. Dicots earlier were considered Monophyletic in origin but present study indicates in the Polyphyletic origin. Monocots are considered as a Holophyletic group. A majority of dicots showing similar characters are grouped in to one branch called the Eudicots or Tricolpates (flowers with 4 or 5 sepals/ petals, tricolpate pollen etc). The remaining dicots are grouped informally into Paleodicots
The APG system of classification proposed in 1998(APG 1,1998)by The systematists B. Bremer, ] K. Bremer, M.W. Chase J.L. Reveal D.E. Soltis, P.S. Soltis and P.F. Stevens . It compromised 462 Families arranged in to 40 Monophyletic orders under a small number of informal higher groups: Monocots, Commelinoids Eudicots, Core eudicots, Rosids, Eurosids I Eurosids II, Asterids, Euasterids I and Euasterids II. Austrobaileyales order which is unassigned in kept at the beginning.
11 unclassified families and four orders with no supra ordinal groups were kept at the beginning of the classification. 25 families of uncertain position were kept at the end of the classification. Judd et al. (1999) made few modifications in APG-1 classification by recognising a total of 51 orders by shifting a few families from informal groups. This classification includes only major families hence nearly 200 families were left out. APG II (2003), a recent revision includes 470 families under 45 orders of which 44 are monophyletic and Austrobaileyales order which is unassigend in kept at the beginning
HEIRARCHY OF APG CLASSIFICATION Recognition of plants based Monophyletic groups called CLADES at all levels. The hierarchy of the classification is as follows- Informal groups Orders Families The informal groups include- Basal Angiosperms, Magnolids, Eudicots, Rosids Asterids. According to APG II ‘Monocots’ is considered Monophyletic group placed under Basal Angiosperms.
CLADE 1: BASAL ANGIOSPERMS They represent a clade that includes following groups- Amborellaceae, Nymphaceae, Austrobaileyales, Ceratophyllales, Monocots. Amborellaceae: the monotypic Amborella trichopoda --the most basal linkage to all basal angiosperms with spirally arranged floral organs, ethereal oil cells. Nympheaceae: 8 Genera, cosmopoliton in distribution and is consistent with the fossil records.generally aquatic in habitat, also found in tropical and temperate regions. The floral diversity ranges from small, simple, trimerous flowers(Cabomba) to large, showy, elaborate flowers(Nympheae, Victoria). Austrobaileyales : represented by Austrobaileyaceae and Trimeniaceae from Australia and recently added Schizandraceae and Illicaceae. Ceratophyllales: based on phytogentic analysis based on rbcL. Ceratophyllum is considered as ‘sister’ of all other Dicots.
MONOCOTS---considered as the major clade among extant angiosperms and also the oldest among the angiosperm fossils comprises of 52,000 species (22% of the total angiosperms.) Orchidaceae and Poaceae are the largest families accounting to 34% and 17%. Ray identified Monocots based on single character ie,. Presence of single cotyledon but phylogenetic studies of non molecular data include 13 similar identifying characters ----single cotyledon, parallel veined leaves, sieve cell plastid with several cuneate protein crystals, secondary vascular bundles in the stem, adventitious root system and sympodial growth. Earlier trimerous condition was considered as unique feature in Monocots, but it is also observed in Nympheaceae and magnolids. APG II considers Monocots as a monophyletic group with 10 orders and 2 unassigned families(Petrosaviaceae and Dasypogonaceae). The orders include – Acorales, Alismatales, Asparagales, Dioscorales, Liliales, Pandanales, Arecales, Poales, Commelianales, and Zingeberales. Molecular analysis shows that Acorus as sister to all other Monocots
CLADE 2:-MAGNOLIDS ‘Primitive Angiosperms’ with 4 orders according to APG II viz. Piperales, Canellales, Magnoliales and Laurales. characterized by the presence of primitive characters like long, broad, net- veined leaves, large flowers, numerous petals , sepals, stamens and carpels, Magnoliales: based on molecular analysis includes 6 families such as Magnoliaceae, Annonaceae, Myristicaceae etc. ----common character i.e. deletion of Apetela 3 gene.& characterized by presence of reduced fibre pits, palisade parenchyma, continuous tectum in the pollen etc. Laurales: includes 7 families characterized by the presence of perigynous flowers, gynoecium embedded in fleshy receptacle, inner staminodes, ascendant ovules etc. Piperales: includes 4 families with characters such as distichous leaves,single prophyll & oil cells. Canellales: includes 2 families viz. Canellaceae and Winteraceae. The family Winteraceae is considered as most primitive extant Angiosperm.
CLADE 3: -EUDICOTS Eudicotyledons (Doyle and Hotton) also termed as Tricolpates or Non- magnolids (represent 75% of the total Angiosperm population) Includes 2 groups‘Basal Eudicots’ and ‘Core Eudicots’. Basal Eudicots: includes five lineages viz Ranunculales, Proteales, Sabiaceae, Trochodendraceae Buxaceae. Core Eudicots: includes seven major clades viz. Gunnerales, Berberidopsidales, Saxifragales, Santanales, Caryophyllales, Rosids Asterids. The molecular data supports the fact that Gunnerales is the sister to the other core eudicots.
CLADE 4: -ROSIDS This clade includes 140 families having common characters like nucellar endosperm , reticulate exine, tricolpate pollen, simple perforations of vessel end walls, mucilaginous leaf epidermis, two or more whorls of stamens, ellagic acid. Molecular analysis reveals two large sub clades i.e. Eurosids I(fabids) and Eurosids II(malvids). A few orders that do not fit into both the sub clades include : Crossosomatales, Geraniales and Myrtales. The Eurosids I include Celestrales, Zygophyllales, Malpighiales, Oxalidales. Other orders like Fabales, Cucurbitales, Rosales, Fagales form the ‘nitrogen fixing clade’. The Eurosid II compromises of Brassicales ,Malvales, Sapindales and Tapiasciaceae.
CLADE 5: - ASTERIDS This is also a large clade with 114 families, 10 orders and 80,000 species. This group is recongnized based on morphological as well as chemical data which include iridoid chemical compounds, sympetalous corolla, unitegmic and tenuinucellate ovules and cellular endosperm.  Previously considered as a polyphyletic group, Asterids are now considered as monophyletic group based on analysis of three genes rbc L, atp B and mat K. Asterids now include four major groups viz. Cornales, Ericales, Euasterids I(lamids) and Euasterids II(campanulids).
Merits of the APG classification The APG classification system strictly adopts the phylogenetic principle of monophyly The APG classification system strictly adopts the phylogenetic principle of monophyly The APG classification system strictly adopts the phylogenetic principle of monophyly The APG classification system strictly adopts the phylogenetic principle of monophylyThe APG classification system strictly adopts the phylogenetic principle of monophyly – reconstructing the phylogeny on the basis of established monophyletic The APG classification system strictly adopts the phylogenetic principle ofThe APG classification system strictly adopts the phylogenetic principle of monophyly .monophyly . It is based on the synthesis of information derived from multiple sources –It is based on the synthesis of information derived from multiple sources – morphology, anatomy, embryology, palynology, phytochemistry, molecular biology,morphology, anatomy, embryology, palynology, phytochemistry, molecular biology, etc.etc. Formal names have been given only to those groups (i. e. orders) where monophylyFormal names have been given only to those groups (i. e. orders) where monophyly has been firmly established.has been firmly established. In order to overcome the problem of paraphyly,In order to overcome the problem of paraphyly, the traditional separation ofthe traditional separation of angiosperms into monocots and dicots has been discarded. Various monocotangiosperms into monocots and dicots has been discarded. Various monocot groups have been placed in between primitive dicots and advanced dicots, e. g.groups have been placed in between primitive dicots and advanced dicots, e. g. Poales are placed in between Magnoliales and Ranunculales.Poales are placed in between Magnoliales and Ranunculales.
Cladograms – evolutionary diagrams - based on morphological and molecularCladograms – evolutionary diagrams - based on morphological and molecular data are presented for showing the phylogenetic relationships, both within anddata are presented for showing the phylogenetic relationships, both within and among the monophyletic groups.among the monophyletic groups. The orders, such as Amborellales, Nymphaeales, Austrobaileyales,The orders, such as Amborellales, Nymphaeales, Austrobaileyales, Chloranthales, which show several primitive features, are placed at the start of theChloranthales, which show several primitive features, are placed at the start of the APG classification.APG classification. The number of unplaced and uncertain families has been gradually reduced fromThe number of unplaced and uncertain families has been gradually reduced from APG-I (40) to APG-III (10).APG-I (40) to APG-III (10). Based on the recent advances in research. The APG classification is increasinglyBased on the recent advances in research. The APG classification is increasingly becoming an authoritative point of reference and a significantbecoming an authoritative point of reference and a significant number of major herbaria, including Kew, Harvard, etc. are arranging their plantnumber of major herbaria, including Kew, Harvard, etc. are arranging their plant specimen collections in accordance withspecimen collections in accordance with APG.
Demerits of APG Classification APG classification is restricted at the taxonomic levels of only order and family. The classification may seem to be sound in theory, but it has still found few followers in practice. Although majority of families or genera have been recognised as monophyletic orders, yet there are several unplaced families or genera in APG – III. The orders have been recognised under informal groups (e.g. Magnoliids, Eudicots, etc), the names of which do not conform to the ICBN.  Angiosperms have been given the rank of a division, followed by the rank of order. In contrast to the previous classification systems, there are no formal taxa between the rank of division and order.
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