Quadrulella (Amoebozoa, Arcellinida, Hyalospheniidae) is a genus of testate amoebae with unmistakable morphology, which secretes characteristic square plates to reinforce the test. They are mainly known from fens and freshwater habitats and have never been documented in deserts. We describe a new species, Quadrulella texcalense, from biological soil crusts in the intertropical desert of Tehuacán (state of Puebla, Mexico). Quadrulella texcalense occurred only at altitudes between 2140 and 2221m.a.s.l., together with the bryophyte genera Pseudocrossidium, Weissia, Bryum, Didymodon, Neohyophyla and Aloina. The soil was extremely dry (moisture of 1.97-2.6%), which contrasts sharply with previous reports for the Quadrulella genus. Single cell mitochondrial cytochrome oxidase I (COI) barcoding of thirteen isolated cells showed an important morphological variability despite having all the same COI barcode sequence. Quadrulella texcalense was placed in a tree containing other Hyalsopheniidae, including a newly barcoded South African species, Q. elegans. Q. texcalense unambiguously branched within genus Quadrulella in a compact clade but with a long branch, suggesting accelerated evolution due to a transition towards a new environment and/or under-sampling.

Departamento de Ecología Evolutiva Instituto de Ecología Universidad Nacional Autónoma de México Circuito Exterior junto al Jardín Botánico C U Coyoacán Ciudad de México C P 04510 Mexico

Department of Zoology Institute of Biosciences University of São Paulo Rua do Matão Travessa 14 101 Cidade São Paulo SP 05588 090 Brazil

Department of Zoology Institute of Biosciences University of São Paulo Rua do Matão Travessa 14 101 Cidade São Paulo SP 05588 090 Brazil; Biology Center Czech Academy of Sciences Branisovska 31 370 05 Ceske Budejovice Czech Republic

Facultad de Estudios Superiores Iztacala Universidad Nacional Autónoma de México Av De los Barrios 1 Los Reyes Iztacala Tlalnepantla Estado de México C P 54090 Mexico

Laboratory of Soil Biodiversity University of Neuchâtel Rue Emile Argand 11 CH 2000 Neuchâtel Switzerland; Botanical Garden of Neuchâtel Chemin du Perthuis du Sault 58 CH 2000 Neuchâtel Switzerland

Laboratory of Soil Biodiversity University of Neuchâtel Rue Emile Argand 11 CH 2000 Neuchâtel Switzerland; Real Jardín Botánico CSIC Plaza de Murillo 2 E 28014 Madrid Spain

Unidad de Medicina Experimental Facultad de Medicina Universidad Nacional Autónoma de México Dr Balmis No 148 Col Doctores Ciudad de México C P 06726 Mexico

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