the rate of aging varies substantially across different species, and this, to a large extent, is genetically based. for example, numerous perennial plants ranging from strawberries and potatoes to willow trees typically produce clones of themselves by vegetative reproduction and are thus potentially immortal, while annual plants such as wheat and watermelons die each year and reproduce by sexual reproduction. in 2008 it was discovered that inactivation of only two genes in the annual plant "arabidopsis thaliana" leads to its conversion into a potentially immortal perennial plant. the oldest animals known so far are 15,000-year-old antarctic sponges, which can reproduce both sexually and clonally.