Volume 3 NOVONNumber 41993Pilocosta (Melastomataceae) Revisited: A New Species, Polyploidy, and the Base Chromosome Number of the Genus Frank AlmedaDepartment of Botany, California Academy of Sciences, Golden Gate Park, San Francisco, California 94118-4599, U.S.A.ABSTRACT. A synoptic review of Pilocosta is pre-sented to facilitate identification and comparison ofa new species, P. nubicola, which is described andillustrated. An original chromosome count of n =11 is reported for this species, together with updatedgeographic and taxonomic notes and a revised keyto the five known species in the genus. Based on anevaluation of recorded chromosome counts, x = 11is established as the base chromosome number ofPilocosta. Pilocosta. one of about 25 genera in the tradi-tionally recognized tribe Tibouchineae, is charac-terized by its quadrate hypanthia and four-merousflowers that are solitary, axillary, and ebracteate. In the paper proposing Pilocosta as a new genus,Almeda & Whiffin (1981: 310) emphasized thedistinctiveness of two collections from Costa Rica'sCordillera de Tilaran that appeared to be related toP. ervthrophvlla. My subsequent fieldwork hasyielded additional material of this rare and little-collected Pilocosta. Analysis of this material and astudy of meiotic chromosomes now provide com-pelling evidence for formal recognition of this entityas a new species described here as P. nubicola. Tofacilitate identification and comparison, it is pre-sented in the context of a brief synopsis with arevised key to the five known species of Pilocosta,a discussion of the base chromosome number of thegenus, and updated taxonomic and geographic notes.THE BASE CHROMOSOME NUMBER OF PILOCOST4 The gametic chromosome number is reported herefor P. nubicola. Meiosis was regular with 11 bi-valents at diakinesis in all cells examined (Fig. IA,B). Bud material for this count was collected froma natural population in the field, fixed in modifiedCarnoy's (Bradley, 1948) solution (4 chloroform, 3ethanol, 1 glacial acetic acid, v v v ), and storedunder refrigeration. Counts were made using a Zeisslight microscope with phase contrast and a 100 xoil immersion objective. Drawings of meiotic figureswere made with camera lucida at a magnification of2100 x. The type collection is the voucher for thiscount. With this report, chromosome numbers are nowknown for all described species of Pilocosta. A countof n = 11 for P. nubicola is in agreement withrecords for P. campanensis (Almeda, 1989) and aPanamanian population of P. oerstedii (Almeda &Chuang, unpublished). It is now clear that P. naona,with populations having n = 22 (Almeda & Chuang,unpublished) and n = 33 (Almeda & Whiffin, 1981),consists of tetraploid and hexaploid races, respec-tively, based on x = 11. The occurrence of n = 11or multiples of this number in four of the five speciesof Pilocosta leaves no doubt that x = 11 is thebasic chromosome number in the genus. Both n =7 and n = 14 were reported for P. erythrophylla(Almeda & Whiffin, 1981). We can now disregardn = 7 as a possible base number in Pilocosta. Ofthe more than 420 species of Melastomataceae forwhich chromosome counts are available, only 13have numbers lower than n = 9. These lower num-bers occur in a diverse assemblage of specializedspecies, which suggests that they have been derivedfrom an ancestral diploid taxon by descending dys-ploidy. NOVON 3: 311-316. 1993.