Evolutionary Biology
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   Sex Allocation
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Schärer Group
Evolutionary Biology
Zoological Institute
University of Basel
Vesalgasse 1
CH-4051 Basel

Sperm competition and cryptic female choice

We are interested in a number of aspects of postcopulatory sexual selection in Macrostomum. Firstly, sex allocation theory predicts that one of the main factors responsible for variation in male allocation is sperm competition. We therefore aim at understanding 1) how male allocation translates into sperm production, 2) which processes influence sperm competition, and 3) if there is any cryptic female choice.

We address the first aspect in three published papers, where we have shown that bigger testes indeed have a higher gonadal activity (Schärer et al. 2004b), that they have a higher sperm production rate (Schärer & Vizoso 2007), and that this plasticity in testis size does not lead to plasticity in sperm size (Janicke & Schärer 2010). We are currently trying to understand if the testis can also modulate the speed of spermatogenesis.

To estimate sperm competition we have developed a number of tools to follow the success of male gametes in the recipient and to look at the resulting paternity patterns. One tool is the so-called sperm-tracking technique (Schärer et al. 2007a), which allows to estimate sperm-transfer success (Janicke & Schärer 2009). Another is classical microsatellite paternity analysis, which allows to estimate sireing success (Sandner et al. submitted). And we are currently evaluating a new tool, which will allow to measure both sperm-transfer and sireing success simultaneously in a non-invasive way, by taking advantage of recently established lines that express green-fluorescent protein (GFP) in the sperm.

As the third aspect, cryptic female choice, was poorly understood theoretically, we have developed some theoretical models to explore the effect of different types of cryptic female choice on sex allocation (Van Velzen et al. 2009). The results clearly suggest that this can be an important factor in the evolution of male allocation (and thus sex allocation). We are currently aiming to test for cryptic female choice by taking advantage of established inbred lines.

Gonadal activity
Immunocytochemical labelling with bromodeoxyuridine (BrdU) allows visualisation of cells in S-phase. BrdU becomes incorporated into cells if, and only if, they are actively undergoing DNA duplication. The number of BrdU-positive cells in the testis can therefore serve as a dynamic measure of male allocation, i.e. testicular activity (Schärer et al. 2004b). It is important to validate that static measures, such as testis size, actually relfect current allocation to that organ with dynamic measures of allocation (see e.g., discussion in Schärer 2009)

BrdU-positive s-phase stem cells (green) in the testis of a labelled Macrostomum lignano
(double-click to see a movie of a confocal laser scanning microscope [CLSM] stack)

Sperm production rate

It is crucial to understand the relationship between allocation to the testis and sperm production. Because worms are transparent, we can estimate the size of the seminal vesicle (which contains the sperm to be used in future copulations). By calculating its size increase in a worm held first in a group of other worms, and then held in isolation for a few days, we can estimate the amount of sperm that is produced in that time (Schärer & Vizoso 2007).

the increase in the seminal vesicle size in an isolated Macrostomum lignano over time can be used as an estimate of the sperm production rate

Sperm tracking

The sperm-tracking approach uses DNA-labelling with a halogenated pyrimidine and localisation of the label using immunocytochemistry. We outline modifications to established protocols to allow visualisation of gametic cells, in addition to somatic cells, determine the duration and patterns of spermatogenesis, and then show that labelled sperm from labelled donors can be observed in unlabelled recipients (Schärer et al. 2007a)

labelled elongating sperm nuclei in the testis of a sperm-labelled donor worm
(double-click to see a movie of a CLSM stack)

labelled sperm in the female antrum of an unlabelled receiver worm
(double-click to see a movie of a CLSM stack)

this page was last updated on Tuesday, December 22, 2015