Senescence in duckweed: age-related declines in survival, reproduction and offspring quality

As they grow old, most organisms experience progressive physiological deterioration resulting in declining rates of survival and reproduction - a seemingly maladaptive phenomenon known as senescence. Although senescence is usually defined with respect only to survival and reproduction, a third component of fitness, offspring quality, may also decline with age. Few studies, however, have assessed age-related changes in offspring quality using measures that truly reflect fitness. In a controlled environment, we tested for age-related declines in three demographic components of fitness (survival, reproduction and offspring quality) in Lemna minor, a small aquatic plant in the subfamily Lemnoideae (the duckweeds) with a short life span and rapid rate of asexual reproduction. Our primary measure of offspring quality, the intrinsic rate of increase, more closely approximates fitness than measures used in previous studies such as size, life span and total reproductive output. We observed strong age-related declines in all three components of fitness: old plants had lower rates of survival and reproduction, and produced lower-quality offspring than younger plants. Theoretical and empirical research on the evolutionary biology of senescence should devote more attention to offspring quality. This often unrecognized component of fitness may change with age - as we have shown in L.minor - and may be shaped by, and feed back into, the same evolutionary forces that give rise to senescence.