Burseraceae

The Burseraceae are a moderate-sized family of 17-19 genera and about 540 species of flowering plants. The actual numbers differ according to the time period in which a given source is written describing this family. The Burseraceae are also known as the torchwood family, the frankincense and myrrh family, or simply the incense tree family. The family includes both trees and shrubs, and is native to tropical regions of Africa, Asia, and the Americas. Just as the family size (in terms of genera and species) differs according to the time period of the study, so, too, does its placement in the higher ranks. Nevertheless, the family is a genetically supported monophyletic group currently and frequently cited within the Sapindales and is recognized as a sister group to the Anacardiaceae. The Burseraceae are characterized by the nonallergenic resin they produce in virtually all plant tissue and the distinctive smooth, yet flaking, aromatic bark,. The origins of the family can be traced to the Paleocene (about 65 Mya) when Beiselia mexicana first diverged in Mexico. The subsequent divergences in the family lineage and migration of the species in the Eocene (53 Mya) from North America have led to the current distributions of the species that are primarily associated with the tropics. Though the family likely originated in North America, the greatest genetic diversity presently is in the Southern Hemisphere. Tabonuco (Dacryodes excelsa) and gumbo limbo (Bursera simaruba) represent the economic, ethnobotanical, and ecological significance of the Burseraceae in the Western Hemisphere, while frankincense (Boswellia sacra) and myrrh (Commiphora myrrha) represent the same in the Eastern Hemisphere. The Burseraceae trees or shrubs are characterized by resins (having triterpenoids and ethereal oils; that are present within the plant tissue from the vertical resin canals and ducts in the bark to the leaf veins. In fact, the synapomorphy of the Burseraceae is the smooth yet peeling or flaking aromatic bark. The clear, nonallergenic resins may smell like almonds, but at least the most well known resins, frankincense and myrrh, have an odor that is distinct from almonds, smelling like incense. The leaves are generally alternate, spiral, and odd-pinnately compound with opposite, frequently long-petiolulate, entire to serrate, pinnately veined leaflets whose symmetry is distinctive in some genera. However, some members are known to have trifoliate or unifoliate leaves. The leaf and leaflet stalks and axis may be brown and scurfy, while the leaf base is swollen and may be concave adaxially. The family members tend to be without stipules. The determinate, axillary inflorescences carry small, radial, unisexual flowers. The plants tend to be dioecious. The flowers may have four or five faintly connate but imbricate sepals with an equal number of distinct, imbricate petals. Also, the stamens, that may contain nectar discs, have distinct glabrous filaments that occur in one or two whorls and in numbers equaling or twice the number of petals; the tricolporate pollen is contained within two locules of the anthers that open longitudinally along slits. The gynoecium contains 3–5 connate carpels, one style, and one stigma that is head-like to lobed. Each locule of the superior ovary has two ovules with axile placentation that are anatropous to campylotropous. The one- to five-pitted fruit is a drupe that opens at maturity. The endosperm is usually lacking in the embryo. Some discrepancy exists in the literature about the size of the Burseraceae. Records say that the family has 17 to 18 genera and 500 to 540 to 726 species. Other authors cite different numbers: 16–20 genera and 600 species; 20 genera and 500–600 species; According to a pollen studies and molecular data, the family is split into three tribes: the Protieae, Bursereae, and Canarieae. The Protieae are composed of Protium (147 species and largest in this tribe), Crepidospermum, Garuga, and Tetragastris. The Bursereae, which are further split into subtribes Boswelliinae and Burserinae, contain Commiphora (nearly 200 species and largest in the family), Aucoumea, Beiselia, Boswellia, Bursera, and Triomma. Finally, the Canarieae are composed of Canarium (75 species and largest in this tribe), Dacryodes, Haplolobus, Pseudodacryodes, Rosselia, Santiria, Scutinanthe, and Trattinnickia. The morphology of the fruit, which is a drupe, helps to distinguish between the three tribes. Though the groupings have slightly changed since the 1990s, the Protieae are described as having a two- to five-parted drupe with either ‘free or adhering parts’ which are ‘not fused in the endocarp’; The Bursereae are described as having a drupe with parts that are fused in the endocarp, but an exocarp with dehiscing valves; and the Canarieae as simply having a drupe with parts that are fused in the endocarp. This is a list of the 19 genera of the Burseraceae with placement in three tribes (and subtribes where applicable): According to the literature, the Burseraceae have not been lumped with other families nor split up into several others. However, they have jumped orders several times. For example, in the early 19th century, the family seems to have been placed in the Burserales, with the Anacardiaceae and Podoaceae. In the mid-19th century and early 20th century, the family was placed in the Geraniales. Then, by the mid- and late-20th century, the family was moved to the Rutales. Finally, in the late 20th century, the family was (and today still is) located within the Sapindales. Families that are consistently found in the same order as the Burseraceae (except when in the Burserales) include the Rutaceae, Meliaceae, and Simaroubaceae. Only in recent studies were the Burseraceae and the Anacardiaceae seen as sister groups. The Sapindales are contained within the malvids of the rosid clade within the eudicotyledons. The order contains nine to 15 families, 460 genera, and from 5,400 to 5,670 to 5,800 species. The currently recognized families include Aceraceae, Anacardiaceae, Burseraceae, Hippocastanaceae, Julianaceae, Meliaceae, Rutaceae, Sapindaceae, and Simaroubaceae. The Sapindales are a clade supported by DNA-based analyses on rbcL, atpB, and 18S sequences. Within the Sapindales are two clades that contain gum and resin: the Rutaceae-Meliaceae-Simaroubaceae clade and the Burseraceae-Anacardiaceae clade. Therefore, the Burseraceae are not the only family with this characteristic. The synapomorphies of the Sapindales include pinnately compound, alternate and spiral leaves that may be palmately compound, trifoliate, or unifoliate, and small four- or five-merous flowers having a characteristic nectar disk and imbricate petals and sepals. Some of these characteristics also occur in the Rosales. However, the Sapindales and Rutales may actually form a complex, since many families jump between them. Indeed, rbcL sequence studies seem to indicate that a sapindalean/rutalean complex exists and may better represent the relationships of the families than the separate orders would. A study based on chloroplast-encoded gene rbcL reconstructed cladograms that include families within both the Sapindales and Rutales. One such cladogram indicated that the Sapindales are robust and that the Burseraceae (and Anacardiaceae) are within a single clade. This grouping seems to make sense as both the Burseraceae and Anacardiaceae have secretory canals in the phloem and resin canals in the leaves, and are unique in the Sapindales for having biflavones in the leaf tissue. However, the two families have several distinguishing characteristics. The resin of the Burseraceae is nonallergenic and two ovules per carpel occur, whereas the resin of the Anacardiaceae can be allergenic or poisonous and one ovule per carpel is found. The Burseraceae-Anacardiaceae clade is sister to a robust cluster of three other families, the Sapindaceae-Aceraceae-Hippocastanaceae clade. The Rutaceae-Meliaceae-Simaroubaceae clade is sister to the Burseraceae-Anacardiaceae and Sapindaceae-Aceraceae-Hippocastanaceae clade. The rbcL technique is supported and considered acceptable until other methods become better developed for the analysis.