PROCEEDINGS OF THE CALIFORNIA ACADEMY OF SCIENCES 
Series 4, Volume 65, No. 2, pp. 41-106, 16 figs., 5 tables, Appendix September 28, 2018 
Louteridium (Acanthaceae: Acanthoideae: Ruellieae: 
Trichantherinae): Taxonomy, Phylogeny, Reproductive 
Biology, and Conservation 
Thomas F. Daniel', Erin A. Tripp 2 
1 Department of Botany, California Academy of Sciences, 55 Music Concourse Drive, San Francisco, 
California 94118, U.S.A.; 2 Department of Ecology & Evolutionary Biology, UCB 334, University of 
Colorado, Boulder, Colorado 80309, U.S.A. E-mail: tdaniel@calacademy.org, erin.tripp@colorado.edu 
Louteridium is treated as consisting of 11 species of mostly shrubs and trees that 
occur from northeastern Mexico to eastern Panama. Nine species occur in Mexico 
and six species are endemic there. Louteridium differs morphologically from other 
genera of the tribe Trichantherinae by its three-lobed calyx, gibbous-throated corol¬ 
la, elongate anther thecae, and large pantoforate pollen. Characteristics of the calyx 
and pollen appear to be unique among Acanthaceae. RADsequencing yielded a high¬ 
ly resolved and for the most part strongly supported phylogenetic hypothesis for 
relationships among species of Louteridium , which itself is strongly supported as 
monophyletic. The three major clades recovered are treated as sections of the genus, 
with one of them newly described as L, section Parcostamium. Several approximate 
chromosome numbers are reported for the genus for the first time. Observations and 
experiments dealing with aspects of floral ecology and reproductive biology reveal 
that the primarily nocturnal flowers share morphological and chemical attributes 
commonly associated with pollination by bats. Both bats and hummingbirds have 
been observed visiting flowers, but pollinators have yet to be documented. Prelimi¬ 
nary conservation assessments result in the inclusion of six species in a threatened 
category (five endangered and one critically endangered). 
Keywords: Morphology, RADsequencing, pollen, chromosome numbers, phenology, 
chiropterophily, endemism 
Louteridium S. Watson is a highly distinctive and geographically restricted Neotropical genus 
of Acanthaceae. Most of its species occur in Mexico, and most of them are rare and/or poorly 
collected. The large, primarily nocturnal flowers show several characteristics of adaptation to 
pollination by bats. Many new field collections combined with recent information on the ecology, 
reproductive biology, and phylogeny of species of Louteridium make the present monograph both 
a timely and comprehensive study of the genus. We also include information relevant to the 
conservation status of each species and offer preliminary conservation assessments for them. 
Watson (1888) described Louteridium based on a collection from Guatemala. He placed the 
genus in tribe Ruellieae and noted three unusual features of the flower: a three-lobed calyx, an 
oblique and inflated corolla, and long-exserted stamens. A few years later, Baillon (1890) provid¬ 
ed a detailed description of a new genus of Mexican plants, Neolindenia, which shared these (and 
other characteristics) with Louteridium, and the two genera are now considered to be congeneric. 
In his comprehensive and influential classification of Acanthaceae, Lindau (1895) treated the 
41 
