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Re: Fern apogamy

> I start with two apogamous species,
> each of which show no genetic variation when grown from spores.  They cross
> and the cross, which should not have been possible, is non-apogamous and
> shows wide variation in the offspring.

Given the premise, yes, it is impossible.

However, cytologically, C. falcatum has three forms: the apogamous triploid, a 
sexual diploid and a sexual tetraploid. Apogamous ferns can hybridize by 
contributing sperm. (The archegonium is suppressed, the antheridia not.) 

A similar situation is described by Lovis in a not-very-easy-for-non-
professionals paper, "Evolutionary Patterns and Processes in Ferns," in 
Advances in Botanical Research, 1977. At length:

"The best known example involves Dryopteris borrerii [now D. affinis ssp. 
borrerii]....This apomictic [apogamous] fern occurs on two levels of ploidy, 
diploid and triploid. It very frequently grows [sympatrically] with the sexual 
tetraploid species D. filix-mas, and can hybridize with it through the medium 
of the antheridia borne on its apogamous prothalli. The hybrid sporophytes (D. 
x tavelii) so produced are tetraploid and pentaploid respectively ..."

Aside, to see if you are following: filix-mas, with a tetraploid complement in 
the sporophtyte produces normally through meiosis spores of diploid complement; 
the resulting fusion in the filix-mas gametophyte archegonium is simply 
additive: 2+2=4 or 3+2=5. Continuing,

"... and can be distinguished from their parents by their robust growth and 
intermediate characteristics. A significant feature of these hybrids is that 
the proportion of 16-mother cell sporangia [normal, sexual] is substantially 
increased (in comparison with D. borrerii) with a consequent diminution of the 
proportion of good spores produced. It appears that the apomictic faculty is 
diluted by the addition of the genomes of the sexual parent."

He goes on to say it is uncertain whether these two hybrids can reproduce 
themselves by apomictic means in nature. In other words, no answer here to your 
basic question.

Going back to the original Cyrtomium hybrid, it would come from either the 
sexual, diploid falcatum and the triploid, apogamous caryotideum giving  
tetraploid progeny OR the sexual, tetraploid falcatum and the same triploid, 
apogamous caryotideum giving a pentaploid set of progeny. Pentaploids are rare, 
but would have to reproduce apogamously and would produce identical offspring. 
You must have the former, then.

Sounds to me like it's not all bad. If your progeny are fertile, you have a new 
strain/species. If they aren't, they have the genes for apogamy.

Tom Stuart

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