Chromosome studies on Brazilian cerrado plants

Cerrado is the Brazilian name for the neotropical savanna, which occurs mai nly in Brazilian Central Plateau, composed of herbaceous-subshrubby and shr ubby-arboreal floras, both of which are heliophilous, highly diverse and re gionally differentiated. Considering species distribution and chromosome nu mbers, some authors have proposed that the herbaceous-subshrubby flora of t he neotropical savanna is quite old, while the shrubby-arboreal flora is de rived from forests, a hypothesis that implies higher chromosome numbers in the savanna than in the forest. If, however, chromosome numbers are similar in the cerrado and in forests, both could be similarly old, indicating tha t bi-directional flow of flora occurred in the past. This paper presents da ta on chromosome numbers and microsporogenesis for 20 species in 13 familie s collected in the States of Sao Paulo, Goias and Minas Gerais, providing p reviously unpublished data for Myrcia (Myrtaceae), Luxemburgia (Ochnaceae) and Hortia (Rutaceae). Meiosis proved to be normal, indicating regularity i n the sexual reproductive process. Chromosome numbers varied from 2n = 18 ( Allamanda angustifolia: Apocynaceae) to 2n = ca. 104 (Ouratea spectabilis: Ochnaceae), being low (20 < 2n < 30) in most of the species. Since similar numbers have been observed with forest species, it is not, at the moment, p ossible to support the hypothesis that cerrado species derived from the sur rounding forests. Instead, we suggest the possibility of a bi-directional e xchange of floristic elements between cerrado and forests during evolutiona ry time.