I recently learned that males of Drosophila bifurca species have gigantic sperm cells, 5.8 cm long and they only produce few hundred such cells during its lifetime. This made me wonder if the male gametes are larger than the female gametes in this species? If so, it would mean we are naming males and females incorrectly in that species, as according to definition, males and females are determined by gametes size [1]. However I cannot find any information about exact size (length of sperm is not enough) or weight of their gametes nor even a picture where both are present next to each other.
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$\begingroup$ Wikipedia definition :P en.wikipedia.org/wiki/Anisogamy en.wikipedia.org/wiki/Sex $\endgroup$– Mateusz DrostFeb 12 at 20:46
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1$\begingroup$ I've honestly never heard of sex solely being defined by the relative size of the gametes. That may be a rule of thumb, but size alone doesn't affect anything downstream, like sexual development pathways. That is determined mainly genetically, and in some species environmental factors such as temperature can play a role. $\endgroup$– MattDMoFeb 12 at 22:00
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$\begingroup$ @David I added link to ncbi.nlm.nih.gov/pmc/articles/PMC5031617 $\endgroup$– Mateusz DrostFeb 12 at 22:27
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$\begingroup$ @MattDMo Yes, sex can be determined genetically or by environmental factors... but then how do you decide which one we would name "male" and which one "female"? $\endgroup$– Mateusz DrostFeb 12 at 22:27
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$\begingroup$ I think there is some confusion in the comments. OP is not talking about sex determination, but rather the very meaning of male and female. The meaning of male and female is typically made in consideration of gamete size, where the female sex produces larger gametes. $\endgroup$– Bryan Krause ♦Feb 12 at 23:15
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1 Answer
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Relative size of gametes
The relative sizes of the sperm and eggs of D. bifurca can be seen in the scanning electron micrograph below, taken from a 2006 Nature paper by Bjork and Pitnick.
The left-hand frame (a) shows a single, 6-cm D. bifurca spermatozoon dissected from the seminal vesicle, where sperm are individually rolled into compact balls. In the right-hand frame (b) six of these have been reduced to the same scale as a D. bifurca ovum and placed alongside it.
It is clear from this that the ovum is much larger than the sperm. I do not think the absolute sizes of sperm and ovum are the point at issue here, but the poster or interested reader can calculate the volume of the ovum from the micrograph above, and that of the sperm from highly detailed electron micrographs in Fig. 6 of a 2008 publication of Dallai et al.. The latter appear to have a cross-section diameter of approx. 0.7–1.0 μm. (Egg volumes for some other Drosophila species can be found here.)
Even if…
Although I think that the section above answers the question, there is a relevant aspect of fertilization in D. bifurca that should be mentioned. This is that, in contrast to Drosophila species with smaller sperm, only a portion of the sperm of D. bifurca actually enters the egg. This is shown in the image below in which only 1.6 mm of the 58 mm long sperm (about 3%) penetrates the egg (only part of which is shown).
(Visualization of sperm in fertilized eggs of D. bifurca. From Karry and Pitnek (1996) Nature 379, 405–6)
Thus, in principle, a giant sperm with a volume of perhaps twice that of the egg would still be able to penetrate and fertilize the latter. The implications of this are considered briefly below.
Implications for definition of male and female gametes
What would the implications be for a Drosophila species in which the volume of the ejaculated sperm exceeded that of the egg, but by an amount that still allowed penetration of a portion containing the nucleus, which could then fuse with the egg nucleus?
Would it mean — as stated by the poster — that “we are naming males and females incorrectly”?
I assume that, like D. bifurca, individuals of this hypothetical species display similar marked sexual characteristics to other species of Drosophila, genetically, physically and behaviourally. In this case, giving the assignation female to what in other species are called sperm (and by implication giving this designation to the individuals from which they originated) seems patently absurd.
I would say that it is the definition that would need changing, not the sex.
Is the definition absolute? As a molecular scientist with a chemical background who last studied organismal biology at the age of about 14, I was surprised at this definition. The paper cited in the question (published in 2016) makes the statement
“Even when there are no separate male and female sexes as such (i.e. simultaneous hermaphrodites), there is dimorphism at the gamete level, where clearly diverged male and female gametes are produced—male gametes being by definition the smaller ones”
It may be thought that the implication is that individuals that produce male gametes are male, and those that produce female gametes are female, but this is not always the case (e.g. simultaneous hermaphrodites).
Judging from the Wikipedia article on anisogamy (the phenomenon of different sized gametes) and popular scientific websites, this seems to be generally accepted by biologists, but I have had difficulty finding when and by whom it was proposed, and what justification was given for the definition. The paper mentioned points back to one by Parker et al. in 1972 but that paper does not even state this explicitly — it is merely implicit in mathematical modelling of the evolution of gamete dimorphism.
I can only suppose that the original definition was based on a need to define male and female gametes in circumstances such as simultaneous hermaphrodites. The definition adopted was consistent with the situation observed in clearly sexually differentiated situations where the male sperm enter the larger female ovum, and so must have seemed a neat solution. However, as we have seen, in Drosophila with giant sperm, only a portion enters the egg, and this is sufficient because only the nuclei of the gametes need to fuse and form a zygote. Biology is not one of the exact sciences, and such a case would not be the first time a definition needed to be tweaked or to admit exceptions.
Footnotes
I thank Professor S Pitnek for directing me to the papers with scanning electron micrographs of the sperm and egg of D. bifurca. If anyone can cast light on the origin of the size-based definition of male and female gametes I will edit this answer to accommodate the information.