Heliotropium is a genus of flowering plants in the heliotrope family, Heliotropiaceae. There are around 325 species in this almost cosmopolitan genus, which are commonly known as heliotropes (sg. ). They are highly toxic to dogs and cats.
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 betrayed 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
Several heliotropes are popular garden plants, most notably garden heliotrope (H. arborescens). Some species are weeds, and many are hepatotoxic if eaten in large quantities 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.
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: Heliotropium sect. Heliothamnus Old World Heliotropium clade Heliotropium sect. Cochranea Tournefortia clade, comprising Tournefortia sect. Tournefortia and all remaining New World Heliotropium species
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.