Distribution and effects of paracentric inversions in populations of Atriplex longipes

Atriplex longipes D rej. ssp. longipes and ssp. praecox (H ülph. ) T uress. are diploid annuals, closely related to other species of the A. hastata group. A. longipes ssp. longipes inhabits marshes with a tall vegetation, and ssp. praecox sea‐shore meadows characterized by a low type of vegetation. The populations of ssp. longipes usually consist of more than 300 individuals, those of ssp. praecox generally of less than 300 individuals. Meiosis and pollen fertility indicate that cytological aberrations, almost exclusively paracentric inversions, occur frequently in the smallest populations of ssp. praecox. Populations exceeding 300 individuals, both of ssp. longipes and ssp. praecox , have a low rate of cytological disturbances. Data obtained from crossing experiments confirm these results. The disturbances observed in meiosis were of the same kind in all crosses, i.e. bridges and fragments formed at anaphase I and II. This is probably due to crossing over within paracentric inversions rather than other types of breakage and reunion phenomena. The correlation between reduction of male fertility and the frequency of inversion bridges in the F 1 hybrids is fairly strong. Only 18% of the crosses between populations of ssp. longipes have reduced male fertility, compared to 80% in the population‐crosses within ssp. praecox . The frequency of inversions might be as much as five times greater within ssp. praecox than within ssp. longipes. The size of the inversions and their effects on crossing over are discussed. The great fluctuations in population size and the occurrence of small populations, at least during shorter periods, are probably one of the main causes of the profound differences in the frequencies of paracentric inversions within A. longipes.