Editing Solidago (section)

==Taxonomy==
''Solidago'' is in the family Asteraceae (formerly known as Compositae), a diverse and widespread clade containing approximately 23,000 species and 12 tribes, which inhabit all continents except Antarctica. Within Asteraceae, ''Solidago'' is in the tribe Astereae and the subtribe Solidagininaeae.<ref name="springer.com">{{Cite book|url=https://www.springer.com/series/1306|title=The Families and Genera of Vascular Plants|language=en}}</ref>

The genus ''Solidago'' is monophyletic as indicated by morphological characters<ref name="Hood-2003">{{Cite journal|last1=Hood|first1=Jennifer L.A.|last2=Semple|first2=John C.|title=Pappus Variation in ''Solidago'' (Asteraceae: Astereae)|date=2003|jstor=41961022|journal=SIDA, Contributions to Botany|volume=20|issue=4|pages=1617–1630}}</ref> and molecular evidence.<ref name="Zhang-1996">{{Cite thesis |last=Zhang |first=Jie J. |year=1996 |title=A Molecular Biosystematic Study on North American ''Solidago'' and Related Genera (Asteraceae: Astereae) Based on Chloroplast DNA RFLP Analysis &#91;microform&#93;. |url=https://www.researchgate.net/publication/35258075 |publisher=[[University of Waterloo]]}}</ref><ref name="Semple-2016">{{Cite journal|last=Semple|first=John|date=2016-05-11|title=An Intuitive Phylogeny and Summary of Chromosome Number Variation in the Goldenrod genus ''Solidago'' (Asteraceae: Astereae)|url=https://www.researchgate.net/publication/303366007|journal=Phytoneuron|volume=2016-32|pages=1–9}}</ref> All ''Solidago'' species are herbaceous perennials, growing from approximately 2&nbsp;cm to 2.5 m tall. Yellow to white, pistillate ray flowers and yellow, perfect disc florets are characteristic of ''Solidago'' inflorescences, which have a wide range of shapes.<ref name="springer.com"/> Molecular studies<ref name="Schilling-2008" /><ref name="Zhang-1996" /> using nuclear rDNA have hypothesized boundaries on the genus ''Solidago'', but there have been difficulties in parsing out evolutionary relationships at the sub-genus scale and defining which should be included and separated from ''Solidago''.

===''Solidago'' and related taxa===
Related Asteraceae genera, such as ''[[Chrysoma]]'', ''[[Euthamia]]'', and ''[[Oreochrysum]]'', have been included within ''Solidago'' at one point or another,<ref name="Anderson-1975">{{Cite journal|last1=Anderson|first1=Loran C.|last2=Creech|first2=Jessica B.|date=1975|title=Comparative Leaf Anatomy of ''Solidago'' and Related Asteraceae|jstor=2441956|journal=American Journal of Botany|volume=62|issue=5|pages=486–493|doi=10.2307/2441956}}</ref> but morphological evidence<ref name="Phytologia-1994">{{Cite journal|date=1994|title=Subtribal Classification of the Astereae (Asteraceae)|last1=Nesom|first1=Guy L.|url=http://biostor.org/reference/131828|journal=Phytologia|language=en|volume=76|issn=0031-9430}}</ref><ref name="Anderson-1975" /><ref name="Rhodora-1981">{{Cite journal|date=1981|title=The Taxonomy of the Genus ''Euthamia'' |url=http://biostor.org/reference/138608|journal=Rhodora|language=en|volume=83|issue=836|issn=0035-4902}}</ref> has suggested otherwise. In a study comparing morphological characters of ''Solidago'' and related subgroups, the authors consider the subjectivity of classifying a genus and how to define it within broader tendencies concerning the taxonomy of North American Asteraceae. Little to no differences were observed between ''Solidago'' and the subgroups in terms of karyotype. However, external morphological characters such as habit, or the general appearance of the plant and how a suite of traits contribute to its phenotype; pappus size; and the point of freeing of stamen filaments from the corolla tube, are useful classification schemes for ''Solidago'', since they are applied to differentiating between Asteraceae taxa. One school of Asteraceae taxonomy thought unites all taxa sharing similar floral head structure and subsequently ignores deviation from this morphology, while another places greater weight on these morphological deviations. The authors argue that the latter opinion should be applied. Since there is no theoretical foundation for relative taxonomic importance of traits, they assert that habit should be a central trait when defining taxa, and subsequently that all the subgroups considered in their study (''[[Brachychaeta]]'', ''Chrysoma'', ''Euthamia'', ''Oligoneuron'', and ''[[Petradoria]]'') should be segregated from ''Solidago''.<ref name="Kapoor-1966">{{Cite journal|last1=Kapoor|first1=B. M.|last2=Beaudry|first2=J. R.|date=1966-09-01|title=Studies on ''Solidago''. Vii. the Taxonomic Status of the Taxa ''Brachychaeta'', ''Brintonia'', ''Chrysoma'', ''Euthamia'', ''Oligoneuron'' and ''Petradoria'' in Relation to ''Solidago''|journal=Canadian Journal of Genetics and Cytology|volume=8|issue=3|pages=422–443|doi=10.1139/g66-053|issn=0008-4093}}</ref>

Results from a leaf anatomy study comparing differences in mesophyll, [[Vascular bundle#Bundle-sheath cells|bundle sheath]] extensions, and midvein structure, among others in a suite of leaf traits,<ref name="Anderson-1975" /> are incongruent with those in an earlier study.<ref name="Kapoor-1966" /> Based on the lack of bundle sheath extensions, it is suggested that ''Chrysoma'', ''Euthamia'', ''[[Gundlachia (plant)|Gundlachia]]'', and ''Petradoria'' should be distinct taxa and outside of ''Solidago''.<ref name="Anderson-1975" /> However, ''Brachychaeta'', ''[[Brintonia]]'', ''Oligoneuron'', ''Oreochrysum'', and ''Aster ptarmicoides'' should be considered as components of ''Solidago''. To summarize, the relation of ''Brachychaeta'' and ''Oligoneuron'' to ''Solidago'' is inconsistent based on these results.<ref name="Kapoor-1966" /><ref name="Anderson-1975" /> Both support the separation of ''Chrysoma'', ''Euthamia'', and ''Petradoria'' from ''Solidago''. A study reviews the taxonomic position of ''Oligoneuron'' relative to ''Solidago'', as based on taxonomic evidence, treats it as separate from ''Solidago'',<ref name="Phytologia-1994" /> similarly to Kapoor & Beaudry (1966). The first molecular phylogeny based on chloroplast DNA treats ''Brachychaeta'', ''Brintonia'', ''Oligoneuron'', and ''Oreochrysum'' as constituents of ''Solidago''.<ref name="Zhang-1996" /> Using consensus trees from ITS data, another study found support for ''Oligoneuron'' as part of ''Solidago'',<ref name="Beck-2004">{{Cite journal|last1=Beck|first1=James B.|last2=Nesom|first2=Guy L.|last3=Calie|first3=Patrick J.|last4=Baird|first4=Gary I.|last5=Small|first5=Randall L.|last6=Schilling|first6=Edward E.|date=2004|title=Is Subtribe Solidagininae (Asteraceae) Monophyletic?|jstor=4135444|journal=Taxon|volume=53|issue=3|pages=691–698|doi=10.2307/4135444}}</ref> and the findings of Zhang (1996). More recently, an analysis of combined ITS and ETS data provided additional support for the inclusion of ''Oligoneuron'' as part of ''Solidago''.<ref name="Schilling-2008" />

Until the 1980s, the genus ''Euthamia'' was largely considered to be a part of ''Solidago'' due to morphological similarities between species in both genera, and a history of synonymy of ''Solidago lanceolata'' and ''Euthamia graminifolia''.<ref name="Rhodora-1981" /> As mentioned, the lack of bundle sheath extensions in ''Euthamia'' compared to ''Solidago'',<ref name="Anderson-1975" /> and deviations in floral morphology<ref name="Kapoor-1966" /> present evidence for separation of these taxa. A taxonomy of ''Euthamia'' as a genus was presented, providing a detailed description of distinguishing external morphological characters, such as fibrous-roots, sessile leaves, and mostly corymbiform inflorescences.<ref name="Rhodora-1981" />

===Evolutionary relationships within ''Solidago''===
Chromosome counts and advances in molecular systematics have enabled greater understanding of evolutionary relationships within ''Solidago''. At the time a taxonomy of ''Solidago'' was published,<ref name="Phytologia-1994" /> related taxa causing contention, such as ''Chrysoma'', ''Euthamia'', ''Oligoneuron'', and ''Petradoria'', were excluded from this genus. The number of ''Solidago'' species has remained relatively stable, around 120, with approximately 80 in North America.<ref name="Semple-2016" /><ref name="Phytologia-1994" /> Due to monophyletic support for the New World taxa<ref name="Beck-2004" /><ref name="Hood-2003" /> and taxonomic difficulties with Old World taxa, the taxonomy provided in the 1990s<ref name="Phytologia-1994" /> only includes North American taxa and thus treats ''Solidago'' as non-monophyletic. Existing molecular-based phylogenies provide monophyletic support for ''Solidago''<ref name="Schilling-2008" /><ref name="Beck-2004" /><ref name="Semple-2016" /><ref name="Zhang-1996" /> given its inclusion of ''Oligoneuron''.

Chromosome counts have proven to be a valuable character in ''Solidago'' taxonomy and in elucidating the cytogeographic history of the genus. Similar chromosome counts may indicate close evolutionary relationships, while different chromosome numbers may suggest distant relationships through reproductive isolation. Chromosome counts have been studied extensively in North America;<ref>{{Cite journal|last1=Cook|first1=Rachel E.|last2=Semple|first2=John C.|date=2008-11-13|title=Cytogeography of ''Solidago'' subsect. ''Glomeruliflorae'' (Asteraceae: Astereae)|journal=Botany|volume=86|issue=12|pages=1488–1496|doi=10.1139/B08-087|issn=1916-2790}}</ref><ref>{{Cite journal|last1=Semple|first1=John|last2=Watanabe|first2=Kuniaki|date=2013-03-02|title=A Review of Chromosome Numbers in Asteraceae with Hypotheses on Chromosomal Base Number Evolution|url=https://www.researchgate.net/publication/48460015}}</ref> all ''Solidago'' species have a base chromosome number of x=9, but the following ploidy levels have been observed: 2x, 3x, 4x, 6x, 8x, 10x, 12x, and 14x.

Though negligible differences in karyotype among ''Solidago'' and related genera were found,<ref name="Kapoor-1966" /> ''Solidago'' taxa with multiple cytotypes are more common than those with one.<ref name="Semple-2016" /> Although chromosome count is a useful metric for differentiating among ''Solidago'' taxa, it may be problematic due to the frequent variation in ploidy levels. Cytogeographic patterns in the ''Solidago gigantea'' complex, with tetraploids occurring in eastern North America and hexaploids in Oregon and Washington, were observed.<ref>{{Cite journal|last1=Semple|first1=John C.|last2=Ringius|first2=Gordon S.|last3=Leeder|first3=Colleen|last4=Morton|first4=Gary|date=1984-07-01|title=Chromosome Numbers of Goldenrods, ''Euthamia'' and ''Solidago'' (Compositae: Astereae). II. Additional Counts with Comments on Cytogeography|journal=Brittonia|language=en|volume=36|issue=3|pages=280–292|doi=10.2307/2806528|issn=0007-196X|jstor=2806528|bibcode=1984Britt..36..280S |s2cid=186241866}}</ref> Cytogeographic patterns are also observed in the ''[[Solidago canadensis]]'' complex: hexaploids within ''S. canadensis'' have been observed east of the Great Plains and are treated as ''[[Solidago altissima]]'', and diploids and tetraploids occurring in the Great Plains are treated as ''Solidago gilvocanescens''. The taxonomic status of ''Solidago ptarmicoides'' created an extensive debate due to frequency hybridization of ''S. ptarmicoides'' with members of the ''Ptarmicoidei'' section of ''Solidago''.<ref name=fna/> It was asserted that ''S. ptarmicoides'' should be united with ''Solidago'' rather than the genus ''Aster'' due to external morphological features such as similar pappus length as well as the same chromosome base (x=9). Information about chromosome number is still a crucial part of current understanding and phylogenies of ''Solidago''.<ref name="Semple-2016" />