Sex and secrets: a story of a missing lady

Males and females are different.  On a biological level, there is no dispute; different chromosomes, different balance of hormones, different reproductive organs.  For many organisms, the differences extend beyond the reproductive organs, to physical differences.  When physical characteristics are dramatically different between males and females of a species, it is termed sexual (male v female) dimorphism (di = apart, morph = shape). Sexual dimorphism can be observed throughout the animal kingdom – the male lion with his majestic mane and the mane-less female, the male peacock with his flashy feathers and the female peacock with her subtle brown feathers, the female black widow and her tiny male counterpart (dinner).  Despite these distinct differences in the appearance of males and females, they still have the same general body shape: whether male or female, both lions look like large cats.  Even the male angler fish, in which is ultimately reduced to a parasite on the intimidating female, still maintains the general body form of a fish until he finds his female.

Every once in a while, mother nature surprises us and breaks from the usual. Sometimes we encounter an organism with such extreme sexual dimorphism that it's difficult to even recognize males and females as the same species.

Drawing of a male Myrmecolacid. Photo credit: Pierce, D. [Public domain], via Wikimedia Commons

Drawing of a male Myrmecolacid. Photo credit: Pierce, D. [Public domain], via Wikimedia Commons

Allow me to introduce you to the Myrmecolacidae, a truly unique family of insects that display extreme sexual dimorphism (with a side of weird sex).  These insects are internal parasites of other insect species for most of their lives. The young larvae offspring find themselves out in the world looking for host before their energy reserves are depleted.  If they find a host, they attach and find a way inside.  Once inside, they will continue developing, feeding off the energy resources of their host.  And here’s where things start to get weird.

The males are found inside ants.  However, unlike most other parasites, the females don’t use the same host as the males.  The females infect crickets, grasshoppers, and mantids.  It is not that these parasites can infect a wide range of insects.  To the contrary, these parasites are highly specific in the species they can infect.  Males only infect ants.  Females only infect crickets, grasshoppers, or mantids (different species are specific to hosts within these options).

    A female infecting a cricket (left) and male infecting an ant (right), indicated by the white arrow.  Photo credit: Jeyaraney Kathirithamby and J. Spencer Johnston

    A female infecting a cricket (left) and male infecting an ant (right), indicated by the white arrow.  Photo credit: Jeyaraney Kathirithamby and J. Spencer Johnston

Somehow males and females of the same species, developed very different host preferences and with those preferences come specializations that allow them to find and survive the unique environments (e.g. external defenses to entry and internal immune system defenses) of those different hosts.  But there is another problem that arises.

How can the males and females possibly meet to mate?  The host ant and host cricket are hardly going to be obliging and bring the male and female together (and they’re both internal parasites in any case).  The only option is to leave the host.

In fact, it is only the males leave their host and develop into a typical adult winged insect body form.  The females stay within their host.  The adult males typically survive for only a few hours once metamorphosed into a free living insect.  Thus, the sole objective of an adult male is to quickly find a female and mate with her.

A male Stylopid species (a close relative with a similar life cycle but different host species) mating with a female inside her bee host.  Photo credit: Colin Boyd

A male Stylopid species (a close relative with a similar life cycle but different host species) mating with a female inside her bee host.  Photo credit: Colin Boyd

You may be thinking, ‘…but you said these insects are internal parasites…and only the males leave their host…how can the male possibly mate with a female when she is inside a host and he outside?’ An excellent question.  And things get weirder.

The female sticks out part of her body and releases pheromones to attract the flying male.  The male, once having detected a female’s scent, will fly to her.  While she is still inside her cricket host, and he is outside the cricket, they will mate. Shortly thereafter the male, having exhausted his short energy supply, will die.  The female, now with fertilized eggs, will take advantage of the mobility of her host to disperse her hundreds of thousands of offspring out into the world.

Clearly these parasites have a complex life cycle.  Despite all that is known about these animals (and in truth it’s not much on the grand scheme), as of 2003, scientists had yet to match a female Myrmecolacid to her male counterpart because of the extreme differences in their body shapes (sexual dimorphism).  The males of many species within this group have been described.  A few females have been described.  But none of these males or females belonged to the same species.

That is, until Dr. Jeyaraney Kathirithamby and Dr. J. Spencer Johnston discovered a female Caenocholax fenyesi in Texas.  In 2003, these researchers collected what were identified as Myrmecolacids both male and female from their respective hosts.  Through careful descriptions of each individual and genetic analysis, they were able to identify a male and female Myrmecolacid of the same species.

The male Caenocholax fenyesi was first identified in 1904.  Ninety-four YEARS later, 94, the first female was found.  For 94 years, we had no idea where to find the female. This identification was only made possible by years of painstaking searching and the use of genetics to compare the DNA sequences of a male and a female.

This strange case of sexual dimorphism in Myrmecolacids raises a myriad of questions about how the sexes of one species differentiate.  How different can the sexes be and still successfully reproduce?  How does sexual dimorphism evolve?

And how often are we just looking in the wrong place for an answer to a mystery?

References:

Kathirithamby, J., & Hamilton, W. D. (1992). More covert sex: the elusive females of Myrmecolacidae. Trends in ecology & evolution, 7(10), 349-351.

Kathirithamby, J., & Johnston, J. S. (2004). The discovery after 94 years of the elusive female of a myrmecolacid (Strepsiptera), and the cryptic species of Caenocholax fenyesi Pierce sensu lato. Proceedings of the Royal Society of London. Series B: Biological Sciences, 271(Suppl 3), S5-S8.