Coulteria delgadoana (Leguminosae, Caesalpinioideae), a new species from the Western Río Balsas Depression, Mexico

Background and Aims: In Mexico, Coulteria is a morphologically diverse genus, having many endemic species in the country. Despite its diversity, species delimitation and circumscription are not fully documented. Botanical explorations in central Mexico during the last 25 years have resulted in the discovery of several new legume taxa in the Río Balsas Depression. A new species of Coulteria from the Western Río Balsas Depression is here described and illustrated. Methods: During fieldwork, morphological studies and electron microscopy analysis of the already species described of Coulteria, we detected a new species in the Infiernillo area of Guerrero and Michoacán, Mexico. For the new species, we estimated its conservation status based on extent of occurrence (EOO) and its area of occupancy (AOO) using the GeoCAT software, applying the categories and criteria of the IUCN Red List. Key results: Coulteria delgadoana, a new species of Caesalpinioideae endemic to the seasonally dry forest of Guerrero and Michoacán (Western Río Balsas Depression), Mexico, is described and illustrated based on morphological evidence. Coulteria delgadoana has long male inflorescences up to 15 cm long, small flowers, leaves with glabrous leaflets and prominent reticulate venation, and a glabrous chestnut brown fruit with purplish tints. Based on IUCN criteria, we suggest an Endangered (EN) category of the species. Conclusions: Morphological characters of the new species C. delgadoana, suggest that it is similar to C. glabra, a species from Colima and Jalisco. The long size of the male inflorescence has not been observed in other Coulteria species. Coulteria delgadoana shares its distribution area with C. lewisii in Infiernillo.


Introduction
Coulteria Kunth as proposed by Lewis (2005), and later confirmed by Gagnon et al. (2013;2016) based on molecular studies, is a genus of dioecious shrubs and trees of the family Leguminosae, subfamily Caesalpinioideae. The flowers typically have a lower cucullate sepal with a glandular-pectinate margin, a flat, papyraceous, indehiscent or late dehiscent (along one suture) fruit, leaflets which are pellucid-punctate, and a style which has a groove in its adaxial suture from base to stigma (Contreras, 1991;Sotuyo et al., 2017). The genus has a distinctive seed chemistry, producing phenylalanine substituted derivatives via the shikimic acid metabolic pathway (Larsen et al., 1975;Kite and Lewis, 1994) and the presence of prismatic crystals in ray cells and chambered axial parenchyma in its wood (Gasson et al., 2009). Coulteria occurs across Central America and Mexico and also in parts of northern South America. In Mexico, Coulteria glabra (Britton & Rose) J.L. Contr., Sotuyo & G.P. Lewis and C. pumila (Britton & Rose) Sotuyo & G.P. Lewis occur across Northwestern Mexico, the first in Colima and Jalisco and the latter in Sonora. Coulteria pringlei (Britton & Rose) J.L. Contr., Sotuyo & G.P. Lewis reaches Northeastern Mexico, from the Sierra Gorda de Xichú, Guanajuato to Tamaulipas, while C. platyloba (S. Watson) N. Zamora can be found with a great morphological variation along the Pacific Coast, from Guerrero to the Tehuantepec Isthmus. Within the Río Balsas Depression and the Tehuacán-Cuicatlán Valley, five or six species occur. In the Yucatán Peninsula, the Caribbean Islands, Central and northern South America, at least three species are found. Morphological variation within Coulteria has always been considered part of broadly distributed species, tolerating a wide geological heterogeneity and growing in a range of dry vegetation types including seasonally dry tropical forest, deciduous woodland and dry thorn scrub (Sotuyo et al., 2017). Our botanical explorations in central and southern Mexico, especially in the Río Balsas Depression, during the last 25 years have led us to reconsider the morphological variation throughout the distribution range, resulting in the discovery of several new legume taxa. The objective of this work is to describe and illustrate a new species of Coulteria from Infiernillo, Michoacán, Mexico, a Protected Natural Area with the category of Biosphere Reserve since 2007.

Material and Methods
The morphological description of the new species was based on macroscopic characters from herbarium specimens (FCME and MEXU, Universidad Nacional Autónoma de México (UNAM), acronyms according to Thiers, 2020) and field collected samples. The specimens examined from Guerrero and Michoacán were compared with the original descriptions and photographs of the different species described in the genus.
Coulteria species are dioecious, female and male flowers occur in separate individuals, and to study flower micromorphology in detail, a JEOL JSM35 (JEOL, Cd. Mx., Mexico) electron microscope (SEM) was used at the Instituto de Ciencias del Mar y Limnología, Universidad Nacional Autónoma de México. Closed buds of the new species were dissected and prepared for SEM by dehydrating in an EtOH series. Pollen grains were prepared for SEM by the standard method described by Erdtman (1952). All samples were dry-mounted on metal stubs using double-sided tape and then coated with gold. The distribution map of Coulteria delgadoana and other closely related Coulteria species was based on curated GBIF data (2020a, b, c), generated using R v. 4.0.2 (R Core Team, 2020) and ggplot2 (Wickham 2016). The conservation status was assessed using GeoCAT software (Bachman et al., 2011) to estimate the extent of occurrence (EOO) and the area of occupancy (AOO) of the species, followed by applying the IUCN (2019) categories and criteria for conservation status assessment.

Results
We recorded a new species of Coulteria from the Infiernillo area (Guerrero and Michoacán, Mexico) in the Río Balsas Depression. The Río Balsas Depression or Río Balsas Basin is a large region that occupies portions of eight states of Mexico (Jalisco, Michoacán, Mexico, Guerrero, Morelos, Tlaxcala, Oaxaca and Puebla). The area is found between the Trans-Mexican Volcanic Belt to the North, the Sierra Madre del Sur to the South and a geological subprovince, the Sierra Norte de Oaxaca to the East. Interrelations between vegetations, evapotranspiration and flows of water in the soil are complex. The Balsas River, especially on its flanks of the Sierra Madre del Sur, contains a diverse endemic biota, some genera have the highest concentration of species in the area (e.g. Bursera Jacq. ex L., Becerra, 2003), clearly linked to environmental conditions, orogeny, volcanism and glaciations. Other endemic species in the Río Balsas Depression can show an amalgam of isolated lineages or species, each occupying different ecological environments (Sotuyo et al., 2007). Coulteria delgadoana Sotuyo & J.L. Contr., sp. nov. is related to Coulteria glabra ( Table 1). The two species are similar in the number of leaflet pairs per pinna and per leaf, as well as in leaflet indumentum and fruit size, but can be distinguished by size of the male inflorescence, and the length of the fruit stipe, amongst other characters discussed below. Coulteria delgadoana is similar to C. glabra, a species from Colima and Jalisco, but it can be distinguished by its longer male inflorescences (generally more than 15 cm long), smaller flowers, its glabrous leaflets with prominent reticulate venation, and its glabrous ochreous-chestnut coloured fruit with violet tints.
Etymology: the new species is dedicated to Alfonso Octavio Delgado Salinas, teacher and friend, a research leader in Mexican botany, and expert in the legume genus Phaseolus L. and other related groups.
Distribution, habitat and phenology: the new species is known only from the Infiernillo region in the states of Guerrero and Michoacán, Mexico. The species grows in deciduous tropical forest, on slopes with rocky soils, and along seasonally streams, at elevations of 77-348 m. Flowering from April to June; in fruit from May to October, the fruits remain attached to the tree until the next flowering season.
Conservation status: based on the extent of occurrence (EOO) estimated at 718,697 km 2 , and the area of occupancy (AOO) of 24 km 2 , we propose a conservation assessment of Endangered (EN) in accordance with IUCN (2019) categories and criteria. If we consider only the area of occupancy, this would result in a higher category of threat because Coulteria delgadoana has a fragmented distribution across two Mexican states and is recorded from only four locations, all subject to the threats from human settlement, road construction and local agriculture.
Hereafter, we present an identification key to distinguish C. delgadoana and its morphologically most closely related species.

Author contributions
SS and JLC contributed to field collection, herbarium revision, analysis of the results and to the writing of the manuscript. JLC processed SEM samples. SS estimated the extent of occurrence (EOO) and the area of occupancy (AOO) for the species.

Funding
This study was funded with the institutional operating budget of SS.