The species names of citrus - a sweet, sour, and sticky mess

Everybody knows the great and tasty citrus fruits - lemon, lime, orange, grapefruit, tangerine, and clementine, and maybe even the more rare bergamot, pomelo, tangelo, kumquat, yuzu, and tangelo.

Various slices of citrus fruits.  (Image from USDA, public domain.)

Since ancient times these plants have been cultivated, domesticated, and used by humans, and hybridization (on purpose or accidental) have created new hybrids. Citrus plants really do like to hybridize and since many of these hybridization and domestication events happened in the past it has been really hard to figure out these questions:
1) What is a species? (biological boundaries and evolution)
2) What is a hybrid? (= cross between two species, biology, evolution, and genetics)
3) What are the parent species of a hybrid? (genetics)
4) What is the correct scientific name for a species or a hybrid? (scientific nomenclature)

With the help of Dr. David Mabberley, an expert on the genus Citrus, I tried to sort this out since so many scientific names for citrus used on beauty and body products are inconsistent and sometimes flat out wrong and need to be updated. Please note that this is a work in progress for science, and that more research is needed to clarify relationships further.  Citrus relationships are a very messy business, indeed. So, here is the most up-to-date information, as far as I can discern.

This is a summary picture of the current hypotheses of the relationships of the most common species and hybrids of Citrus:

Origins and scientific names for the most commonly cultivated citrus fruits. 
Based on data from D.J. Mabberley's publications.

In botanical taxonomy a species (*) can only have one current and unique scientific name.  Other names are synonyms and their use should be phased out, and only be listed as synonyms in publications. This is to ease the communication and understanding in botany, horticulture, ethnobotany, and agriculture. We should all know what we talk about and we should talk about the same thing when we use a species name, right?
(* except for some fungi, but the mycologists are sorting that out now too, slowly. )

Hybrids either get their parents' names with an 'x' in between parent names (mother listed first), or a brand new species epithet preceded by a 'x'.  So, in the figure above, the name for orange can be listed as Citrus maxima x Citrus reticulata, or Citrus x aurantium.  You often see the name Citrus sinensis or Citrus x sinensis for oranges, but those are synonym names that should not be used anymore.

The group Citrus x aurantium includes a large array of different cultivars and varieties, such as orange, tangor, grapefruit, bitter orange, clementine and tangelo.  This is because the initial hybrids where then back-crossed to different parents, so Citrus x aurantium is really a large hybrid complex. 

So, what are then the most up to date names for cultivated and commercial citrus fruits? Here they are:

Common name	Current scientific name	Common inaccurate botanical and ingredient names
bergamot	Citrus × limon	Citrus aurantium var. bergamia, Citrus bergamia
bitter orange	Citrus × aurantium	Citrus amara, Citrus aurantium ssp. amara, Citrus iyo
cedrat	Citrus reticulata	Citrus x reticulata
citron	Citrus medica	Citrus medica var. acida
clementine	Citrus × aurantium	Citrus clementina
grapefruit	Citrus × aurantium	Citrus paradisi, Citrus × paradisi
kaffir lime (now called makrut lime)	Citrus hystrix
kumquat	Citrus japonica
lemon	Citrus × limon	Citrus limon, Citrus medica var. limon
lime	Citrus × aurantiifolia	Citrus aurantifolia
makrut lime	Citrus hystrix
mandarin	Citrus reticulata	Citrus x reticulata
orange	Citrus × aurantium	Citrus aurantium, Citrus aurantium var. sinensis, Citrus sinensis, Citrus × sinensis
pomelo	Citrus maxima	Citrus grandis
satsuma	Citrus reticulata	Citrus unshiu
Tahiti lime (seedless)	Citrus x latifolia
tangelo	Citrus × aurantium	Citrus tangelo
tangerine	Citrus reticulata	Citrus x reticulata,  Citrus × tangerina
tangor	Citrus × aurantium	Citrus nobilis, Citrus x nobilis
yuzu	Citrus × junos	Citrus junos

Many of the commercial, non-commercial, government or non-profit databases and publications do not use these updated names.  They are often lagging behind and changing names take time.

Want to read more about the delicious history and taxonomy of citrus plants?  Try these scientific papers if you want something more substantial than Wikipedia. 

Mabberley, D. J. 1997. A classification for edible Citrus (Rutaceae). Telopea 7: 167-172.

Mabberley, D. J. 2004. Citrus(Rutaceae): A review of recent advances in etymology, systematics, and medical applications. Blumea 49: 481-498 

 Zhang, D. & D. J. Mabberley.  Citrus in Flora of China. [Lots of interesting information here!]

Li, X., R. Xie, Z. Lu, & Z. Zhou. 2010. The origin of cultivated Citrus as inferred from internal transcribed spacer and chloroplast DNA sequence and Amplified Fragment Length Polymorphism fingerprints. Journal of the American Society for Horticultural Science 135: 341-350. [Note - some of their results do not agree with the parent hypotheses presented above.]

(Many thanks to Dr. David J. Mabberley for expert help and information for this post.)

LUSH: Kaffir Limes and Lime trees (Example 1-4)

LUSH with its fragrant, colorful, and funky beauty products uses a lot of natural, plant-based ingredients.  They provide excellent information about their ingredients, but still manages to list many species and/or their Latin names wrong in their Ingredient Finder.  Their ingredient list goes mostly by plant names, not plant ingredient names. I applaud their efforts in ingredient transparency, and these examples illustrates some of the issues raised in the blog posts below this.

LUSH gets many species names correct, but here are some examples of botanical inaccuracies when plant species are listed:

Kaffir lime leaf, Tilia europaea:  The photo at this entry shows leaves of the Kaffir lime plant, Citrus hystrix, a relative of the common lime and orange, and in the Rutaceae family.  The text under this entry refers to an European hardwood tree genus called limes or linden trees, that is related to mallows and cotton and in the Malvaceae family. The listed species, Tilia europaea, should be listed as Tilia x europaea (common lime), since it is a naturally occurring hybrid of two wild European species in this genus (a synonym for this species is Tilia x vulgaris).
    This is a complete mix up of two species, one a kind of tropical Citrus and the other a tall tree from Europe. These two species couldn't be more different.  Lime of course, also refer to calcium oxide or calcium hydroxide, among many other things.
    Common names in English are often not unique to one particular plant species, that is why we have unique Latin names for all species. I think that the species used in the products are Citrus hystrix, which has plenty of essential oils of a flavor that are also used in Asian cooking. But you shouldn't have to guess, it should be clear on the company's website.  Here is how different the two species are in leaf morphology:

Kaffir Lime leaves, Citrus hystrix
(Image source: Fatrabbit, Wikimedia commons, CC license)

Common Lime leaves, Tilia x europea
(Image source: Alvesgaspar, Wikimedia commons, CC license)

Blackcurrant, Ribes Nigrum: should be Ribes nigrum.

Flax, Linum Usitatissimum: should be Linum usitatissimum

Indigo Henna (Indigofera Tintctoria): should be indigo, Indigofera tinctoria. Note spelling of the Latin name.  Henna comes from another plant, so the made up common name 'indigo henna' is confusing and doesn't refer to a species, but to a LUSH product.