Re-arranging Limbs on the Family Tree – When is an Agave not an Agave?

The botanical world has long been a tug of war between the taxonomic world of lumpers and splitters. Lumpers prefer to combine as many plants as they can into a single genus or species, while splitters prefer to categorize in the opposite direction, creating new genera and species when they feel the science dictates. We have been closely following the tug of war as it relates to one of our favorite genera, Agave (century plants).

In the mid 1990s, three papers (Bogler and Simpson 1995, 1996) and (Hernández-Sandoval 1995), pushed for combining the genera Manfreda, Agave, Prochnyanthes, and Polianthes (tuberose) into a single genus, Agave. These papers showed that a DNA-based family tree (phylogeny) showed the genera Manfreda, Prochnyanthes, and Polianthes were genetically nested in the middle of the genus Agave, which is not allowed. This change would also eliminate the hybrid genus xMangave, of which we have been pioneers. The current prevailing trend in taxonomy is to only use molecular DNA, with little or no attention paid to conventional taxonomy, which includes morphology and phenology (how they look and grow). Below are photos of two of the genera proposed to be merged into Agave. As you can see, they look nothing like what we know as a typical Agave.

Let’s equate the agave situation to humans. You are at a family reunion with relatives, all of which are related, either closely or distantly. Currently, everyone in the room has one of four last names; Smith (agave), Jones (manfreda), Williams (polianthes), and Johnson (prochnyanthes). Everyone takes a DNA spit test, and from that, you construct a family tree. The largest group are the Smith clan (agave), but the family tree (phylogeny) shows that most of the Smith clan sorts out on one limb of the family tree, called a clade. A few Smith clan, however, are hanging out on other branches of the family tree, near, but not mixed with the Jones (manfreda), Williams (polianthes), and Johnsons (prochnyanthes). In taxonomy, this is called a paraphyletic tree, which means the relationships are not correctly aligned. To fix the family tree into more closely related groups (monophyletic), you have two choices.

If you’re a lumper, you would choose to have the entire group to change their name to Smith. In taxonomy, this would be known as Smith sensu lato (Smith in the widest sense). The other option would be to take the outliers named Smith that showed up mixed with the Jones, Williams, and Johnsons, and give them each a new last name which reflects where they show up on the family tree, and how they are related to each other and to the original four groups. This would leave members of the Smith family with less members, but those members would more accurately reflect the relationships shown by DNA. This option, known as splitting, would be referred to as Smith sensu stricto (Smith, in the strict sense).

In 2012, German taxonomist and agave expert, Joachim Thiede, a lumper, officially moved all plants from the genus Manfreda, Polianthes, and Prochnyanthes into the genus Agave. In 2020, Ofelia Jiménez-Barron etal. published another DNA paper, again, showing that these three former genera previously made the genus Agave a fake unit (polyphyletic). Thiede, followed up Jimenez-Barron’s paper, also in 2020, with a complete revision of the genus Agave. Like all bandwagons, almost all of the top botanic gardens in the world jumped on this one, without any regard to the physical, visual, and growth differences between the former three genera. Manfreda are the only genus with purple spotted leaves, all without teeth. Polianthes are deciduous and have narrow fleshy leaves and no spines. Prochnyanthes are also fleshy with no spines. Agaves, however, are all evergreen and have spines

Finally in January 2024, a new paper by J. Antonio Vazquez-Garcia, etal., looked at the same problem through a different lens, and to what we feel is a much better approach using the same data. He looked at which agave species created the “nesting” problem that caused Thiede to combine Manfreda, Polianthes, and Prochnyanthes into Agave. If these species were removed from the genus agave, then Manfreda, Polianthes, and Prochnyanthes could remain valid genera. It turned out that the Agave species that were the problem had already been cited many times over the previous century as probably imposters, not actually belonging to the genus Agave, due to significant morphological differences. Those genetic outliers, include the group of porcupine leaf agaves, the tropical Agave ellmeetiana, and the spineless Agave bracteosa.

Vazquez-Garcia proposed what we think is a much more logical solution, by splitting up the genus agave and moving the problems species into three newly created genera, thereby solving the paraphyletic problem with Manfreda, Polianthes, and Prochnyanthes nesting within Agave in the family tree. Agave bracteosa, which is an oddity in the genus with no spines, as well as being the oldest branch on the family tree (6.18 million years ago) becomes a new genus, Paleoagave. The group of twelve needle-like species, all which cluster together genetically, and most of which don’t die after flowering, become a new genus, Echinoagave. These include Agave stricta, striata, tenuifolia, albopilosa, etc. Finally, the tropical species, also with no terminal spine, Agave ellemeetiana, becomes the genus Paraagave.

But now, we need more names

This now makes the agave family into seven monophyletic genera, and restores the validity of genera Manfreda, Polianthes, and Prochnyanthes, as well as the hybrid genera xMangave. It also causes the need for more names for hybrids with the three displaced agave groups.

Several years ago, we introduced our cross of the former Agave striata with Agave lophantha. Now, this becomes a bigeneric hybrid between Agave and Echinoagave, so will need a new nothogeneric name, which we christen as xClosetoagave. Then there are crosses like Agave ‘Mateo’ (bracteosa x lophantha), which becomes a bigeneric hybrid of Agave and Paleoagave, which we’ll call, xSortofanagave. Our hybrid ‘String Bean’ and ‘Straight and Narrow’ are crosses between the former Agave striata and the former Agave bracteosa. These will now be known as xNotquiteanagave. Finally, our hybrid of Agave victoriae-reginae and the former Agave bracteosa will become xOwhatanagave.

If that’s not enough, we also need to re-name the former xMangave ‘Man of Steel’, which is a hybrid with Agave macroacantha, Manfreda maculosa, and the former Agave striata, which now becomes a tri-generic cross of Manfreda, Agave, and Echinoagave, which we’ve named xMenageatroisavara. We hope this makes a bit of sense and offers a bit some insight into the world of plant taxonomy and the constant plant name change conundrum.

Bibliography:

Bogler DJ, Simpson BB. 1995. A chloroplast DNA study of the Agavaceae. Systematic Botany 20: 191−205.

Bogler DJ, Simpson BB. 1996. Phylogeny of Agavaceae based on ITS rDNA sequence variation. American Journal of Botany 83: 1225−1235.

Bogler DJ, Pires JC, Francisco-Ortega J. 2006. Phylogeny of Agavaceae based on ndhF, rbcL, and ITS sequences: implications of molecular data for classification. Aliso 22: 313−328.

Hernández-Sandoval L. 1995. Análisis cladístico de la familia Agavaceae. Boletín de la Sociedad Botánica de México 56: 57−68.

Ofelia Jiménez-Barron, etal., 2020, Phylogeny, Diversification Rate, and Divergence Time of Agave sensu lato (Asparagaceae), a Group of Recent Origin in the Process of Diversification, Frontiers in Plant Science

Thiede Joachim, 2012, Nomenclatural transfers from Manfreda Salisb., Polianthes L. and Bravoa Lex. to Agave L. (Agavaceae/Asparagaceae). Haseltonia 17: 94–95. 2012 94

Thiede, Joachim, 2020 Agave Agavaceae Monocotyledons 21-311

Vázquez-García, J.A., C.S. Rosales-Martínez, J. Padilla-Lepe, G. Hernandez-Vera, y L.J. García-Morales. 2024. New genera and
new combinations in Agavaceae (Asparagales). Phytoneuron 2024-02: 1–14. Published 15 January 2024. ISSN 2153 733X