STERILITY IN THE NORTH-AMERICAN LAKE CRESS NEOBECKIA-AQUATICA (BRASSICACEAE) - INFERENCES FROM CHROMOSOME-NUMBER

Sterility of lake cress results from uncertain factors and may be a si gnificant element in the decline of populations over the past century. The inability of lake cress (Neobeckia aquatica) to produce viable se eds restricts its dispersal to vegetative fragments which are transpor ted less effectively over long distances. We obtained mitotic counts o f 2n = 24 for individuals from seven populations of lake cress, a spec ies for which the chromosome number was unreported previously. In cont ext of chromosome number distribution in the Brassicaceae based on lit erature reports for 192 mustard genera, the base number of tribe Arabi deae (in which lake cress is placed) and of all genera presumed to be closely related to lake cress is x = 8. The presence of 24 chromosomes indicates that lake cress is a triploid derived from an x = 8 chromos omal series. Highly sterile triploid hybrids (2n = 3x = 24) have also been reported in several genera related to lake cress. The extremely w ell-developed system of vegetative reproduction in lake cress may part ially compensate for its sexual sterility. The discovery that lake cre ss is triploid offers a specific explanation for its sterility and dis closes special considerations for the conservation of this rare specie s.