Table 1.

Summary statistics of lethal phenotypes of young genes. (I) Gene age was described in (8); age groups 0 to ~3 My and 3 to ~6 My were pooled to increase sample size. A gene was considered essential for viability if it was constitutive RNAi lethal (8); fertility is not a subject in this study. (II) Lethality stage of “pupal” includes all substage categories, such as prepupal, early pupal, late pupal, and pharate. “Before pupal” includes multiple larval stages, including early larvae and late larvae. “Other” includes mixed-stage lethal, stage unknown, or stage undefined.

I. Proportion of essential genes (constitutive RNAi lethal)
Young genes
Age (My)	Essential genes	Nonessential genes	Subtotal	Proportion of essential	p*
0–6	4	9	13	31%	1.00
6–11	25	51	76	33%	0.78
11–25	13	30	43	30%	0.60
25–35	17	46	63	27%	0.24
Total	59	136	195	30%	0.31
Old genes
Age (My)	Essential genes	Nonessential genes	Subtotal	Proportion of essential	Age (My)
>40	86	159	245	35%	>40

II. Stage of lethality
	Young genes		Old genes
Pupal	47	80%	43	50%
Before pupal	6	10%	38	44%
Other	6	10%	5	6%
Total	59	100%	86	100%
P†	0.0009

^*Two-tailed Fisher’s exact test P for essential/nonessential young genes in each age group compared with those for old genes;

^†Two-tailed Fisher’s exact test P for pupal/non-pupal lethals for young genes compared with those for old genes.