Figure 15. Increased egg retention protects offspring viability and fertility under mild environmental stress.

(A) Effects of osmotic stress on survival of embryos from eggs developing ex utero in the strain JU751. Eggs were extracted from adult hermaphrodites (mid-L4 +36 hr) by dissection and exposed to variable concentrations of NaCl (and M9 control condition) for 15 hr. Embryonic survival was estimated by counting the fraction of live larvae 24 hr later. N=120–288 eggs per treatment. (B) Differences in total lifetime offspring production of selfing JU751 animals derived from surviving eggs developing ex utero (dissected from adults at mid-L4 +36 hr) versus in utero when exposed to mild osmotic stress (0.3 M NaCl) for 15 hr; in parallel, ex utero eggs (dissected from adults at mid-L4 +36 hr) were exposed to a control treatment (M9 buffer) for 15 hr. 24 hr after the treatment, larvae from each of the three treatments were allowed to develop and reproduce for 3 days and scored for total offspring production. Animals derived from eggs developing in utero showed significantly higher fertility than animals derived from eggs developing ex utero when exposed to osmotic stress (Kruskal-Wallis Test, χ²=4.66, df=2, p=0.03). N=18–19 animals per treatment. (C) Differences in developmental time of JU751 animals derived from surviving eggs developing ex utero (dissected from adults at mid-L4 +36 hr) versus in utero when exposed to a low concentration of acetic acid (1 M) for 15 min; in parallel, ex utero eggs (dissected from adults at mid-L4 +36 hr) were exposed to a control treatment (M9 buffer). Eggs were then allowed to hatch and develop for 45 hr, at which time we determined their developmental stages. Each stage was assigned a score of development as follows: L3=1; early mid-L4=2; midL4=3; lateL4=4; Adult=5. Each dot represents the mean score reached by offspring produced by one single mother (N=30) mothers per treatment, producing between surviving 9–57 larval offspring. Animals derived from in utero eggs had a significantly higher mean developmental time score, that is they exhibited accelerated development compared to animals derived from ex utero eggs (Kruskal-Wallis Test, χ²=38.93, df=1, p<0.0001). (D) Differences in total lifetime offspring production of selfing JU751 animals derived from eggs developing ex utero versus eggs and L1 larvae in utero when exposed to a low concentration of acetic acid (1 M) for 15 min; in parallel, ex utero eggs (dissected from adults at mid-L4 +36 hr) were exposed to a control treatment (M9 buffer). Note that L1 larvae directly exposed to 1 M acetic acid died immediately. Twenty-four hr after the treatment, larvae from each of the four treatments were allowed to develop and reproduce for four days until cessation of reproduction; total offspring production was then scored 24 hr later. There were no significant differences in mean fertility between the four different treatment groups (Kruskal-Wallis Test, χ²=4.00, df=3, p=0.26). N=15 animals per treatment. Eggs were dissected from adults at mid-L4 +36 hr and L1 larvae at mid-L4 +48 hr.