Genus of flowering plants in the borage family Boraginaceae
Heliotropium is a genus of flowering plants traditionally included in the family Boraginaceae "s.l.", but placed in the family Heliotropiaceae within the Boraginales order, by the Boraginales Working Group.. There are around 325 species in this almost cosmopolitan genus, which are commonly known as heliotropes. They are highly toxic to dogs and cats, as well as to humans.
Etymology.
The name "heliotrope" derives from the old idea that the inflorescences of these plants turned their rows of flowers to the Sun. Ἥλιος ("helios") is Greek for "Sun", τρέπειν ("trepein") means "to turn". The Middle English name "turnsole" has the same meaning.
A Classical myth, told in Ovid's "Metamorphoses", imagines that the water nymph Clytie, in love with the sun god Helios, was scorned by him. Wasting away, she transformed into the heliotrope, whose flowers supposedly always face the Sun.
Morphology.
Like other members of the Heliotropiaceae, plants in the genus "Heliotropium" have 5-merous, tetracyclic flowers and actinomorphic corollas. They likewise share in their characteristic terminal styles and highly modified stigmatic heads (basal stigma, infertile apex). Species in the genus are typically herbs or subshrubs exclusively and are characterized by their dry fruits that divide into two or four mericarpids. 
Ecology and human use.
Heliotropes are popular garden plants, most notably garden heliotrope ("H. arborescens"). Some species are weeds, and many are hepatotoxic if eaten due to abundant pyrrolizidine alkaloids. There have been cases of canine death due to over-ingestion of this toxic plant. Some danaine butterflies, such as male queen butterflies, visit these plants, being attracted to their pyrrolizidine alkaloids. Though it is not palatable and most animals will completely ignore it, there have been cases of horses, swine and cattle being poisoned due to contamination of hay.
Caterpillars of the grass jewel ("Freyeria trochylus"), a gossamer-winged butterfly, feed on "H. strigosum".
The sap of heliotrope flowers, namely of European heliotrope ("H. europaeum"), was used as a food coloring in Middle Ages and Early Modern French cuisine.
One of the most famous ragtime piano melodies is "Heliotrope Bouquet", composed in 1907 by Louis Chauvin (the first two strains) and Scott Joplin (the last two strains).
Garden heliotrope is grown in Southern Europe as an ingredient for perfume.
The purplish facial rash of dermatomyositis is called "heliotrope rash" because it resembles "E. arborescens".
Heliotrine and heliotridine.
Seeds of the "Heliotropium" genus were discovered in the 1940s and 50s to be responsible for liver disease in populations that consumed them in large quantities, either inadvertently (as a contaminant of food crops) or deliberately (associated with the ingestion of herbal infusions for the treatment of certain ailments). The seeds contained high concentrations of pyrrolizidine alkaloids, identified mainly as the N-oxide of heliotrine (74%), and one or two other compounds similar in character to lasiocarpine. More recently, in 1993 and 2017, there have been reported cases of poisoning in Tajkistan from wheat contaminated with "Heliotropium lasiocarpum" seeds.
Taxonomy.
Taxonomic revision supported through molecular phylogenetics led to the recognition of "Euploca" as genus separate from "Heliotropium". In contrast, the genus "Tournefortia" was included in "Heliotropium" in a 2016 revision.
Within "Heliotropium", there are four major clades:
Origins of diversification.
Three of the four major clades within "Heliotropium" have their centers of diversity in South America. The origins of the remaining Old World "Heliotropium" clade can be traced back to a single colonization event from the New World. ITS1 data shows there is a single characteristic long deletion between positions 61 and 111 in the genome of the Old World species, which defines the Old World "Heliotropium" species and separates them from their New World counterparts. Researchers concluded this is a single autapomorphic character from a single deletion event in the past. This most reasonably explains how the whole group may have come to share this characteristic deletion when comparing the genomes of Old World and New World "Heliotropium".
The most likely driver of "Heliotropium" diversification across the three New World clades is early Andean uplift. Researchers identified three independent diversification events in the phylogeny of Andean "Heliotropium", whose timings correspond to late Miocene Andean uplift as well as the development of arid environments in South America during the Pliocene. These three diversification events each mark the separation of the "Heliothamnus", "Cochranea", and "Tournefortia" clades from the rest of "Heliotropium".
"Heliothamnus" diversification is estimated to have taken place in the late Miocene. The age of "Heliothamnus" suggest that its diversification could have been triggered directly by the uplift of the Andes, something that would have promoted speciation in inner-Andean valleys and the Andean scrub. The majority of endemic "Heliothamnus" taxa in the region are restricted to these sorts of environments, further supporting this theory as the current leading theory explaining "Heliothamnus" diversification.
Before the main rise of the Andes, "Cochranea" and "Tournefortia" coinhabited the Andean region at the same time and significant speciation had not yet occurred. Once the Andes began to rise, "Cochranea" became isolated on the western side of the Andes while "Tournefortia" grew on the eastern side. This east-west division is still true of each group’s present distributions. The rise of the Andes affected the climate of the region and is believed to have contributed to the hyperaridity of the Atacama Desert, something that could have acted as an additional barrier to filter out other "Heliotropium" species into the range of "Cochranea", thus promoting "Cochranea" speciation. Elevation differences would have also acted as barriers that helped promote speciation in "Tournefortia" species as many large groups within "Tournefortia" became well-adapted to high-elevation environments while other "Heliotropium" clades did not and thus could not coinhabit the same environments as "Tournefortia".
References.
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