B chromosomes and fertility in a native population of Hymenachne amplexicaulis (Poaceae: Panicoideae: Paspaleae)

Fertility is a measure of reproductive success, defined as the capacity of an individual to reproduce and generate offspring, and has been identified as an important trait correlating to the invasive ability of a species. Hymenachne amplexicaulis (Rudge) Nees is a grass native to South America of high bromatological quality, making it a species with good forage potential. In the places where it was introduced it is now considered an invasive weed due to its dominance over native vegetation. In this work, individuals of H. amplexicaulis were analyzed from a native population in its southernmost native range, situated on Yacyretá reservoir dam. All individuals show a diploid cytotype with two B chromosomes (2n = 2x = 20 + 2B). This is the first chromosome count recording B chromosomes for the genus, and a new cytotype for the species. The karyotype was found to be symmetrical and unimodal, composed by 20 metacentric chromosomes and two small B chromosomes. Pollen viability was tested at 90.6%, seed set production of population reaches 18.65%, and the viable seeds presented a germination power of 41%. The reproductive performance is less than expected with respect to the mean amount of spikelets that a single inflorescence possesses and notwithstanding optimal conditions of supply and availability of viable pollen for fecundation. Germination test indicate good seed quality and viability in native range. In H. amplexicaulis effective fertility is lower than its potential fertility, but it is not an indicator of absence of invasive threat. Reproductive performance of B host plants indicates sufficient invasive potential.