Males evolved from the dominant isogametic mating type

Cultures and induction of gamete formation Eight heterothallic strains of Pleodorina starrii [S1] were used in this study and were maintained at 20-22°C as described previously[S1]. Two heterothallic strains (2000-602-P14female and 2000-602-P15male) were used in the initial stage of the present study. Later, other isolates of P. starrii from a different collection (2001-608-P26female, 2001-608-P21male, and 2001-608-P17male) [S1] and their F1 progeny (2005-701-F1-5female, 2005-701-F1-1male and 2005-701-F1-3male) were also used. To induce sperm packet formation, actively growing cultures of a male strain in 24 ml Pleodorina-growth medium[S2] were concentrated to ~0.4 ml by centrifugation, diluted with 10-15 ml nitrogen-free Pleodorina-mating medium [S2], and placed at 22-25°C on a 14:10 L:D cycle.