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Heliotropium

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Heliotropium

Heliotropium /ˌhliəˈtrpiəm, -li-/ is a genus of flowering plants with around 325 species commonly known as heliotropes. They are highly toxic to dogs and cats, as well as to humans.

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.

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:

Although traditionally included in the family Boraginaceae s.l., the genus is placed in the family Heliotropiaceae within the Boraginales order by the Boraginales Working Group.

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.

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