Ficus seeds can germinate in date palms because the environment within a date palm's fruit cluster provides the necessary conditions for germination, including moisture, warmth, and a suitable substrate for the seeds to root in; essentially, the date palm acts as a "nurse tree" offering a protected microclimate for the fig seeds to sprout, especially if the seeds happen to be deposited there by birds. In addition to a moist & sheltered environment, decaying organic matter in date palm fruit cluster also provides nutrient source for developing fig seeds.

When I began studying figs (Ficus) with horticulturist Steven Disparti over 30 years ago, I thought each species of fig had it own unique, symbiotic pollinator. This may still be true for some of the 876 species of Ficus; however, multiple pollinators and evidence from host & pollinator sharing data have complicated the fig & fig wasp story. The relationships undoubtedly follow kinships within subgenera & sections depicted in enormous Ficus DNA cladograms (evolutionary trees). There are 4 pollinated Australian fig species on campus in subgenus Urostigma and at least 3 verified species of Australian fig wasps: (12 possible interactions). If you exclude Section Conosycea & wasp Eupristina, and only consider the more closely related Section Malvanthera & 2 Pleistodontes wasp species on campus, there would be 7 possible interactions. Another complicating factor introduced into seedling diversity is possible hybridization between fig species pollinated by multiple wasp species each coming from at least 2 different host fig species. In fact, some authorities have speculated that host-sharing and host-switching by fig wasps may have lead to the enormous number of fig species. In host-sharing multiple fig species share the same pollinator wasp, while in host-switching a pollinator wasp evolves to use a different fig species as its primary host.

Carlos A Machado, Nancy Robbins, M Thomas P Gilbert, & Edward Allen Herre. 2005. "Critical review of host specificity and its coevolutionary implications in the fig/fig-wasp mutualism." Proc Natl Acad Sci U S A. 2005 Apr 25; 102 (Suppl 1):6558–6565. doi: 10.1073/pnas.0501840102

Gang Wang, et al. 2023. “Genomic evidence of prevalent hybridization throughout the evolutionary history of the fig-wasp pollination mutualism.” https://doi.org/10.1038/s41467-021-20957-3

Another Australian strangler fig (Ficus watkinsiana) is closely related to F. rubiginosa. In fact, some DNA cladograms show the 2 species as sister clades. Its native Australian pollinator wasp is Pleistodontes nigriventris. This fig has been introduced to Kauai, Hawaii, where it is pollinated by P. imperialis. An impressive specimen of Ficus watkinsiana is the centerpiece of the "Monkey Trails" habitat at the San Diego Zoo. According to the Zoo's online Fig Web Page, it produces seedlings; therefore, pollinator wasps must be present. Could the pollinator be the rustyleaf fig wasp (P. imperialis)?

The term "drupelet" refers to the minute fruits of banana fig that line the inner walls of syconium. Some references refer to them as "fruitlets." They are ripened ovaries from female flowers. The mature drupelet has an inner layer called endocarp that makes them technically different from achenes. References calling them achenes are incorrect according to fig authorites Ira J. Condit & William B. Storey at UC Riverside. If the female flower was pollinated the endocarp layer would typically enclose a minute, viable seed. Most of the F. pleurocarpa syconia I examined were empty (hollow) and seedless, indicating lack of pollination. The center of mature syconia was occupied by pollen-laden male flowers (anthers). At this time the female flowers were no longer receptive. Unfortunately, I never observed a F. pleurocarpa syconium with receptive female flowers.

Could F. pleurocarpa at Palomar be self-pollinated? All the references I have consulted state that this species exhibits obligate mutualism: It must be pollinated by one or more fig wasps of the genus Pleistodontes. I have never observed any fig wasps in it's syconia when anthers were shedding pollen (when female flowers appeared to be past their receptive stage). Figs are typically protogynous: I.e. female flowers receptive before male flowers (anthers) shed pollen. I must conclude that pollen would typically be carried into syconium from another fig tree (possibly host-sharing). I am not aware of any other insects that could accomplish pollination or induce seed production without pollination.

There may be other factors involved in seed formation in Ficus pleurocarpa; however, as I have stated above, the explanation remains an enigma in my opinion.

My colleagues have offered some hypotheses to explain seeds in Ficus pleurocarpa syconia at Palomar College.

1. In some figs (Ficus carica) the syconia and ovaries of female flowers develop by parthenocarpy, which means they form enlarged (ripened) syconia containing seedless drupelets without fertilization by fig wasps. Parthenocarpic varieties of common edible figs are commonly sold at nurseries. This process is different from parthenogenesis, when an unfertilized egg develops into an embryo within a seed. The latter mechanism comes under the heading of apomixis that is very unlikely in figs according to the literature.

2. Self pollination: This can be ruled out because F. pleurocarpa is clearly protogynous; i.e. the female flowers mature before male flowers. By the time anthers are shedding pollen the female flowers are no longer receptive. This strategy favors cross pollination.

3. Activity of other minute arthropods inside syconia, possibly spread by birds, resulting in pollination of some receptive female flowers. This would explain the paucity of seeds in F. pleurocarpa syconia.

4. Partially eaten syconia by birds possibly resulting in other insects entering syconium and pollinating female flowers. I have examined syconia with no external damage that contained seed-bearing drupelets with viable seeds.

5. Pollination of some receptive flowers in Ficus pleurocarpa syconia by fig wasp (e.g. Pleistodontes) from another campus fig species who happens to enter receptive ostiole. Host-sharing has been documented for Pleistodontes and a large F. rubiginosa grows near greenhouse location for banana fig. The female wasp may be unwilling or unable to lay her eggs and she is absorbed by syconium enzymes (ficin) by the time it matures, leaving no evidence of fig wasp. This also explains the paucity of seeds. In my opinion this hypothesis fits the principle of parsimony basic to all science and tells us to choose the simplest scientific explanation that fits the evidence and peer-reviewed literature.