Table 1.

Comparison of reproductive isolation (RI) factors for two pairs of inter-species hybrids.

RI feature	C. remanei × C. latens a	C. briggsae × C. nigoni b
Extrinsic isolation (temperature-sensitivity)	n.d.	Present
Gametic isolation	n.d.	Strong
F1 male inviability	Weak/Moderate	Strong
F1 female inviability	Weak/Moderate	Strong
F1 male sterility	~10% or ~95%	~100%
F1 female sterility	~0%	~40% or ~60%
F2 male inviability	Strong	n.a. c
F2 female inviability	Strong	n.a. c
Haldane’s rule	Sterility and inviability	Sterility and inviability
Darwin’s corollary	Stronger RI for maternal C. remanei	Stronger RI for maternal C. briggsae
Heritable VRI d	Present	Present

^athis study;

^b(Woodruff et al. 2010; Kozlowska et al. 2012);

^cF₂s cannot be produced owing to complete F₁ male sterility;

^dgenetic variability within species for reproductive isolation between species