Reproductive systems and low outbreeding barriers between Jacaranda cuspidifolia and J. mimosifolia (Jacarandeae, Bignoniaceae)

The genus Jacaranda shows notable karyotype stability and a prevailing
self-sterile breeding system with evidence of late-acting
self-incompatibility in several species. However, some studies have
indicated self-compatibility in J. mimosifolia, a species cultivated
worldwide in tropical and subtropical areas. Jacaranda
cuspidifolia is a close related species with natural distribution
broadly overlapping to that of J. mimosifolia, and manual heterospecific
pollination studies have indicated that these species are interfertile,
but there is no report on the breeding system of the former. In this
study, we used hand-pollination experiments, pistil longevity,
epi-fluorescence and histological analysis of post-pollination events to
determine the breeding system of J. cuspidifolia. We also employed intra
and interspecific crosses and seed germination tests to reevaluate the
breeding system in J. mimosifolia and the inter-fertility between
the two species. Some fruits were initiated from self-pollinated pistils
in J. mimosifolia, but none of them reached maturity. On the other hand,
complete absence of fruit development by self-pollination was verified in
J. cuspidifolia, while in situ pollen tube growth and
histological analysis of post-pollination events in selfed pistils
revealed the characteristic ovule penetration, fertilization and endosperm
initiation, as observed in other bignoniaceous species with late-acting
self-incompatibility. Low outbreeding barriers seem to operate between
these species because reciprocal interspecific crosses and hybrid seed
germination tests indicated they are bilaterally interfertile. However,
fruit/seed production and seed germinability were significantly lower when
pistils of J. cuspidifolia were pollinated with pollen of J.
mimosifolia, compared with crosses in the opposite direction, which
indicates a partial unilateral incompatibility. This result is discussed
in the contest of the possible occurrence of self-compatibility of J.
mimosifolia. The low level of incongruity operating between the two
species also points to their recent evolutionary divergence.