Gynodioecious mating system aids in tracking gene flow in papaya

Commercial papayas in Hawaii are gynodioecious, segregating for female and

hermaphrodite plants, with hermaphrodites being preferred for production purposes.

Genetically engineered (GE) papayas with unique marker genes and resistance to papaya

ringspot virus (PRSV) were released in Hawaii in 1998. A survey of GUS marker

transgene expression in 628 spontaneous plants from non-agricultural environments on

Oahu in 2010 and Hawaii Island in 2011 revealed that 22% were GE. Sex segregation in

the population of escaped transgenics indicates that seed is the main means of transgene

dispersal from commercial fields, not pollen. A higher ratio of hermaphrodites to

females (1.83 H : 1 F) than expected (1.41 H : 1 F, assuming five generations of

panmictic mating between 2000 to 2010 and equal fitness of sexes) indicates that feral

transgenic populations are mostly first generation escapes from commercial fields and are

poor long term competitors in non-agricultural environments. Among longer established

non-transgenic escapes, a lower than equilibrium H:F ratio (1.24 H : 1 F) indicates that in

non-agricultural settings females are more fit than hermaphrodites, which function

primarily as relatively inefficient pollinizers, rather than as seed producers in their own

right. PRSV resistance conferred little advantage for transgenic plants in feral

populations, since incidence of disease was very low (6%). Penetration of transgenes

into feral populations is predicted to be gradual and dependent upon continued

commercial production of GE cultivars.