GENETIC DIVERSITY IN MEDITERRANEAN DIPLOID AND TETRAPLOID BROMUS L (SECTION BROMUS SM) POPULATIONS

The levels of genetic diversity assessed from allozyme data were inves tigated in 25 populations of Mediterranean Bromus intermedius, B. squa rrosus, B. lanceolatus, and B. hordeaceus from Algeria. The geographic ally restricted diploids B. intermedius and B. squarrosus displayed le ss genetic diversity (the mean population gene diversity of Nei (H-u) ranged from 0.03 to 0.12) than the widespread tetraploid colonizers B. lanceolatus and B. hordeaceus (H-u = 0.07-0.27). Deviations from Hard y-Weinberg expectations in diploid populations of B. intermedius and B . squarrosus were observed owing to heterozygote excess at several loc i and suggested that these self-fertilizing species may have substanti al amounts of allogamy. Tetraploid populations of B. lanceolatus and B . hordeaceus were largely homozygous at homologous loci and frequently exhibited intergenomic fixed heterozygosity in accordance with their alloploid origin. Genetic variation at the infraspecific level was mos tly distributed within populations in the four species, B. hordeaceus showing the lowest level of interpopulation differentiation (G(st) = 0 .06) and the highest level of gene flow (Nm = 3.75). Consistent gene f lows are in agreement with the strongest intercontinental invasive beh aviour of B. hordeaceus. Less differentiation was reported in the lite rature among later introduced B. hordeaceus populations from England a nd Australia, indicating reduced differentiation under the process of colonization. Moderate divergence occured among the four taxa, with in terspecific genetic identities ranging from 0.87 to 0.93. In spite of substantial genetic similarity, species were clearly differentiated, w ith each tetraploid being more closely related to a diploid: B. hordea ceus to B. squarrosus and B. lanceolatus to B. intermedius.