Shifts in postembryonic somatic ploidy levels in Daphnia pulex

Although the presence of polyploid cells in somatic tissues of the cladoceran Daphnia has been reported, the nature of ploidy level shifts through life has not been investigated. This study examined the effects of growth and age on ploidy level variation across 10 instars in Daphnia pulex. The transition from mitotic to endomitotic cycles occurred during embryogenesis in all tissues, but the timing of it appeared to be tissue-specific. Lower than expected DNA estimates for the 4C and 8C ploidy classes suggested an underreplication of the genome during the first two endomitotic cycles, while complete replication seemed to occur in subsequent cycles. Polyploid cells underwent from four to seven endomitotic cycles over the 13 postembryonic instars, with ploidy levels reaching a maximum of 2048C in the labrum. The number of polyploid cells in all tissues, except the digestive tract, remained uniform beyond the fourth instar. Three patterns of ploidy level increments were observed i n the various tissues. Polyploid cells in the epipodite and rostrum exhibited age-related shifts, reaching asymptotic levels, while those in the digestive tract similarly reached asymptotic levels but then re-entered the mitotic cycle to enable tissue growth. Finally, ploidy level shifts in both the labrum and the epidermis were affected by age and food levels.