My citrus are in bloom so have done the following cross pollination attempts. I placed the mother tree first and the pollen donor second:

Reale Kumquat x Star Ruby Grapefruit
Reale Kumquat x Unknown Mandarin
Lane Late Navel Orange x Reale Kumquat
Unknown Mandarin x Reale Kumquat

The unknown mandarin is marked as "mandarino hybrido" and I can't find any further information about it.

The Lane Late and Unknown Mandarin are overloaded with flowers so I doubt the ones that I cross-pollinated will be held by the tree to maturity. I'll try cross-pollinating some more when they become available to increase the chances of having one that develops into a mature seed bearing fruit.

I have higher hopes for the Reale Kumquat holding its flowers to mature fruit since it doesn't have many.

The Reale Kumquat x Unknown Thai Pummelo fruitlet is hanging on and seems to be growing very slowly.

Does anyone grow Lane Late Navel Orange? I read that it is seedless but does anyone get seedy fruit? I'm hoping that if I'm lucky enough to get a mature fruit from the cross-pollinated flower, the presence of seeds indicates a successful cross-pollination. I'm also hoping the seeds will give me some zygotic seedlings.

I'll attempt to cross-pollinate some Star Ruby Grapefruit flowers when I see some that are ready. I realise however that grapefruit seeds are highly nucellar and probably won't produce any zygotic seedlings, but it's worth a try nonetheless.

Very interesting crosses. Especially, at least for me, Reale kumquat x Star Ruby grapefruit. I haven't heard about an existing hybrid like that. I've read Reale kumquat is a cross of 'Marumi' kumquat and the calamondin. Its' cross with the grapefruit would bear unusual fruits, I think.
Keep us informed about your hybridisation attempts.

PS. I also try to cross citrus. A year ago I pollinated a flower of Ponderosa lemon with Rangpur lime's pollen. Ponderosa lemon is highly monoembryonic (~95%), now the seeds are sprouting. Hope there will be some hybrids I will be able to identify on the basis of leaves' shape.

Although making citrus hybrids can be fun, normally almost all crosses result in fruits that are of inferior quality to their parents,- (but rarely a superior fruit such as the Eureka Lemon is brought about). - Millet (624-)

Wirtual24 thanks, I will keep this thread updated with any progress. Your cross sounds very interested too, keep us updated also.

Millet, yes this is mainly for fun. If by some fluke I get a good quality fruit that would be great but it's not expected.

As expected my trees dropped most of their flowers including 7 of the 8 I cross pollinated. Only 1 cross pollinated flower remains - Reale Kumquat x Star Ruby Grapefruit. A fruitlet is forming, fingers crossed it will grow to maturity.

My other cross pollinated fruitlet of Reale Kumquat x Unknown Thai Pummelo is still immature. It's been a year and one month now on the tree. I'm hoping the coming warm weather will finally allow it to grow fully and ripen.

That's it for cross pollination attempts this year. Hopefully next year will be better.

Update: Both the Reale Kumquat x Star Ruby Grapefruit fruitlet, and last year's Reale Kumquat x Unknown Thai Pummelo fruitlet have fallen off. There were no seeds in the later, it was still immature.

There might be one last cross pollination I can make this year which is an extreme long shot: Bear's Lime x Star Ruby Grapefruit.

My new 1 year old Bear's Lime tree (about 8 inches tall) has 1 flower forming, and to my surprise the Star Ruby Grapefruit is flowering for the second time this year with 3 new flowers forming. I'll attempt a cross pollination with the Bear's Lime as the mother tree. I doubt such a small young tree could successfully carry any fruit to maturity, and on top of that Bear's Lime very rarely forms seeds. Worth a try though since I have to wait till next year for any new attempts with my other citrus.

I pollinated the Bearss Lime flower with the pollen from a Star Ruby Grapefruit flower tonight. The Bearss Lime flower had no anthers formed at all at the ends of the filaments, which looked very odd.

Now it is highly unlikely that any seeds will be formed, but I can live in hope, just like playing the lottery